What is speaking in tongues and what does it prove?
In Acts 2:5-12 the apostles were speaking in a way that people from 15 different nationalities were able to understand the speakers in their own tongue. Since the hearers heard a language that they understood, it stands to reason that the language of tongues was not unintelligible. It would appear that its primary purpose was to expand the number of people who could understand and not just baffle or mystify listeners. According to I Corinthians 14:22, the whole purpose of speaking in tongues is to impress unbelievers. That is certainly what happened in Acts 2. If the apostles were speaking gibberish, then the charge that they were drunk would have been understandable.
All of the groups that claim to have the ability to speak in tongues, speak something that is nonsensical or is, at the very least, no known language. Often it follows a rhythm, but it has no language construct. Occasionally someone will claim they are speaking in the language of angels. The problem then is: how would a non-believer know or even be impressed with someone speaking the language of angels, if there was no one available to verify whether the speakers were speaking that language or were faking it?
Anyone can make up gibberish. I have had some people tell me that they go to tongue speaking services and make up the stuff themselves just so they won't feel out of place. Of course, they could never admit it to their group because that would then be "proof" to the others that they were not believers. As far as these people (the fakers) knew, no one else had been able to tell that they were faking it. Since they have a kind of self-imposed silence about it and some were faking, how could anyone tell if they were all faking and just too scared to admit it? It is so much like the little kid's story, "The Emperor's New Clothes".
The ultimate thing to realize is that tongue speaking was serving a purpose in New Testament times. Paul says in I Corinthians 13:8, "Love never ends. As for prophecies, they will pass away; as for tongues, they will cease; as for knowledge, it will pass away." There is a time limit to at least prophecies, speaking in tongues and knowledge. It is not clear from this passage if these are meant to represent all of the gifts (as listed in I Corinthians 12:8-11) or if it is just these three that have a time limit. All three of these particular gifts are associated with the revealing of the mind of God. For example, the apostles used the gift of tongues on the day of Pentecost and it helped to get the message out to people who otherwise would not have heard it as quickly. Prophecy (revealing the mind of God) and miraculous knowledge (knowing things for which you should not know, e.g., Peter with Ananias and Sapphira in Acts 5) are all things that deal with knowledge or learning of various types.
This makes complete sense when we think in terms of the revealing of the Word of God, which was the topic at hand. Jesus told his disciples that they would be guided into all truth (John 16:13). What happens when all the truth is revealed? Then the process of revealing is complete or at an end. If nothing is left to reveal, then what need is there of a continuous process to reveal?
Ultimately, the conclusion is that tongue speaking, as it was done in the days of the New Testament, does not match what is called tongue speaking today. It was a known language, used for the purpose of impressing unbelievers and to spread the word to different nations. It was said that it would end and all indications are that all requirements for it ending occurred many centuries ago.