It's hard to give a specific answer because you haven't given enough information.

Which instruments are fading out?

Explain the steps you are taking to create the MP3..

As you probably know...MP3 music is compressed sounds and some instruments like cymbals or high frequency sounds will suffer in fidelity... this can be rectified to some degree by increasing the bit-rate of your MP3...but the higher the bitrate the bigger the file...so you have to work out a good compromise depending on the type of track you have.