I think it's .mp3 and .wav
Because all the songs in Warcraft III uses .mp3 while ambient sound , unit sound and misc is .wav
It uses looping, that will re-play the sound/music over and over again given a certain time for that sound/music to end.
Assume that we use Credits.mp3 as music it has a duration of 00:01:23 (can be seen via Sound Editor) and I have created a test map for you.