This is a good question. 👍 Direct evidence for "speaking in tongues" being viewed as anything other than existing human languages is lacking at this time.
Having said the above, it should be pointed out that many historians & scholars of the 19th century made a distinction between various kinds of tongues. For example, Philip Schaff writes the following:
"Irenaeus (Adv. Haer. 1. v. c. 6, § 1) speaks of "many brethren" whom
he heard in the church having the gift of prophecy and of speaking in
"diverse tongues" (Παντοδαπαῖς γλώσσαις), bringing the hidden things
of men (Τὰ κπύφια τῶν ἀνθπώπων) to light and expounding the mysteries
of God (τά μυστήρια τοῦ θεοῦ). It is not clear whether by the term
"diverse," which does not elsewhere occur, he means a speaking in
foreign languages, or in diversities of tongues altogether peculiar,
like those meant by Paul. The latter is more probable. Irenaeus
himself had to learn the language of Gaul. Tertullian (Adv. Marc. V.
8; comp. De Anima, c. 9) obscurely speaks of the spiritual gifts,
including the gift of tongues, as being still manifest among the
Montanists to whom he belonged. At the time of Chrysostom it had
entirely disappeared; at least he accounts for the obscurity of the
gift from our ignorance of the fact. From that time on the glossolalia
was usually misunderstood as a miraculous and permanent gift of
foreign languages for missionary purposes. But the whole history of
missions furnishes no clear example of such a gift for such a
I would argue that free vocalization forms of speech such as "jubilation" can be considered indirectly as a type of species of glossolalia. A good place to do some scholarly research on the linguistic nature of "jubilation" is John C. Poirier's "Tongues of Angels: Concept of Angelic Languages in Classical Jewish and Christian Texts." Excerpts from the book can be found here.
Also, check out the Roman Catholic scholar, Eddie Ensely's "Sounds of Wonder, 20 Centuries of Praying in Tongues and Lively Worship." He points out that jubilus, was connected in the fourth through ninth century in the early church with the repetition of certain psalms. It consisted of a melismatic and prolonged final syllable singing of the alleluias. Historians note that this improvised form of prayer, which flourish on the final syllable of alleluia, might continue for up to five minutes long. It was understood in the early church as a type of reflection of the angelic chorus.
This practice of jubilation continued in both the west and east for many centuries. For example, as late as the 14th century, meaningless syllables called teretismata can be seen to be introduced into the Greek manuscripts of Byzantine chant. For further research in the area, see Robert Webber’s The Complete Library of Christian Worship (StarSong, 1994): “A Brief History of Jubilation” (280-308).
Although spiritually induced, jubilation appears to be a natural phenomena similar to secular non vernacular forms of speech such as yodeling, jazz scat singing and euphonious yo-ho type work songs. A close religious parallel to the ancient practice of jubilation is that of African-American forms of spiritual moaning melodically dressed with melismas, having a unison and heterophonic tone.
When one examines the nature of the gift of jubilation in church history it appears to be identical to how certain 19th century European exegetical scholars viewed the gift of tongues.
For example, the 19th century Lutheran commentator George Stoeckhardt writes in his "Exegetical Lectures on the First Epistle of Paul to the Corinthians" a note on 1 Corinthians 14:2-5. In that section, he describes the gift of tongues. He uses almost identical terms to describe the gift of jubilation in the early patristic period:
"There were various kinds of speaking in tongues. One who had this gift would, moved by the Spirit, with his tongue bring forth pleasant sounds or songs, unintelligible,...the Spirit moves the inner spirit of man to utter euphonious sounds..." (Commentary on 1 Corinthians, pages 73 & 83)
Poirier points out that, in the ancient Jewish pseudepigraphal books, there are numerous historical references to a liturgical jubilus. This jubilus mimicked the esoteric tongues of angels. Poirer points out that St. Jerome was known to have interacted with Jewish believers in Antioch and lived near Bethlehem. So, a strong case can be made that the jubilus Jerome describes, coincides with the description of metaphorical angelic tongues described in Jewish pseudepigraphical writings. This type of speech is also mentioned by the Apostle Paul in 1 Corinthians 13.
St. Augustine writes:
What is jubilation? Joy that cannot be expressed in words. Yet the voice expresses what is conceived in the heart and cannot be explained in words. This is jubilation...Where speech does not suffice… To manifest his joy, the man does not use words that can be pronounced or understood, but bursts forth into sounds of exaltation without words...they break out into singing on vowel sounds, that through this means the feeling of the soul may be expressed, words failing to explain the heart’s conceptions. (Commentary on Psalms)
We also know that this practice of jubilation was widespread in the early church and continued for centuries. For example, the Benedictine theologian Rupert of Deutz (1075-1130 A.D.), writes of the devotional prayer practice of jubilation that continued in his day:
We jubilate rather than sing, and extend a short syllable over several neums or groups of neums, in order that the spirit may be moved by the beautiful sounds...
The most plausible explanation, for why the ancient practice of jubilation was not commonly identified as being the same as praying with the New Testament gift of tongues, is that it was often done in a manner that was different than a prophetic speaking in tongues.
The Apostle Paul did not want the practice of speaking in tongues to be done in a public communal manner. For example, the Apostle writes:
If therefore the whole church be assembled together, and all speak
with tongues and there come in men unlearned or unbelieving, will they
not say that you are mad? (1 Corinthians 14:23)
It is a reasonable inference that, during the first couple hundred years after the church was established, a new name was given to describe the communal practice of chanting or singing in tongues. There were a couple of benefits that resulted from this name change in the patristic period.
First of all, potential controversy in the church that this practice was in violation of 1 Corinthians 14:23 was eliminated.
Secondly, by downplaying the phrase "gift of tongues (glossolalia)" the use of the phrase "jubilation" avoided controversy from cynics. After all, if Christians really had the same gift as was given on the day of Pentecost then why did they need to study other languages - so as to translate Scripture for other people groups? As a case in point, Eusebius notes (8:14) that Apollonius claimed to know the languages of all humans and all animals - which led him to ask why he then needed the services of a translator on his travels. It is reasonable to argue that cynics might say the same thing about Christians who made claims to speaking in tongues.
Finally, a new name like jubilation helped to demystify the experience so as to better relate it to the Greek and Roman converts who were used to such free vocalization in other contexts.
The above reasons account for why St. Augustine, St. Chrysostom and others did not claim the gift of jubilation was the same as the gift of tongues described in the New Testament on the day of Pentecost.
In our modern era there are cases where phenomena of name swapping and branding have taken place. For example, the word "apple" was originally understood as a description of fruit in general. Another example, in the field of fishing, is that the renaming of certain species is sometimes done to encourage customers and increase popularity - e.g. slime head is now orange roughy, and Patagonian toothfish sells better as Chilean sea bass.
The same sort of name swapping has happened in the field of viticulture. For example, the grape varietal Zinfandel was long considered unique to America. However, it was only recently that DNA profiling was used to confirm that it is not so unique after all. Rather a Croatian grape varietal, by the name of Crljenak kaštelanski, has been found to be genetically identical to the Zinfandel grape.
There are many other cases in life in which name changes have been made so as to increase brand appeal. What happened historically with the fishing industry, the wine industry, and other areas of life, is arguably analogous to what likely happened with the gift of tongues in Corinth and the gift of jubilation found in post-New Testament times.
For a defense of the position that there were different kinds of tongues in a linguistic sense in the New Testament era, see this article by the Roman Catholic lay apologist Dave Armstrong.