Unlike the ambient_generic which automaticly looks for the *.wav files in the "/sound" directory. The target_mp3audio starts in the main mod directory.
To target your sounds you have to use the following: sound/yoursound.mp3
Sounds not playing when using more then 1 target_mp3audio's:
The target_mp3audio has a problem if you don't give it a target name, usually if you don't have/want to trigger it you don't give it a target name, true?
The solution: If the fade distance of let's say 2 target_mp3audio's overlap each other, only the first which was added in the editor is played, the other is muted somehow. This also happens if you check the flag "don't fade volor". Now if you give all of them a different target name they will play their sound, and it doesn't seem that the sounds have to use a certain type of encoding