Look at the MP3 files and are they in some very high bit rate?
You didn't tell enough for folk to guess if it's that.
i recently bought the lg spirit phone. i have tried numerous times to play music from it, it will work for a bit then stop again. i have formatted and reformatted the SD card...loaded the music from the computer...tried different ways from loading the music to the card...even directly to the phone. it will work for a day then tell me file not supported by music player. i have changed the music player from the original to rocket which is the highest rated on the android market. what else can i do to get the music to play...