target_mp3audios are really buggy.
for me the "start on" flag just doesnt work 95% of the time. what i do to trigger mine is have them triggered by a multimanger which is triggered by a trigger_auto that targets the multimanager after 1 second after the map loads.
other problems that occur are that if one mp3_audio overlaps another (in terms of the fade distance) one of them may not play. the solution to this is to give them different names, although once again you maybe going

and
also another suggestion is to set the volume to 200 instead of 255.
however you may find that after all of that some still dont play. i had this on my map. for some reasion i decided to reload the map and then a couple of the sounds that werent playing before were now playing. so..uuurrrghg DAMM YOU MP3_AUDIO!!! DAMM YOU!