I went along with my pentecostal wife to a couple of church sessions where they were speaking in tongues. She was adamant afterwards that it must be real as she couldnt have done it. I could easily imitate the sounds and showed her and she never mentioned it again.
Anyway here are some videos. I sure there are much better ones out there and many are made by sceptical people imitating the sounds rather than being under the influence, but to my ear they sounded like what I heard so it will give you an idea
http://revival.aimoo.com/Speaking-in-Tongues/Speaking-in-tongues-videos-on--1-408083.html
In my experience (predominately Black evangelical churches), the persons “speaking in tongues” are not trying to be understood, and no one thinks they ought to be understood. Their vocalizations are merely considered evidence of being possessed by the holy spirit.
however in many other churches it is considered that if the glossonalia isnt “translated” then it is wrong - there is some verse in the NT that alludes to this but I am too lazy to look it up , one of Pauls letters I think