The true answer is to start off with something better than an mp3 to begin with, but the best way to get good sound on a bog standard 68020+ amiga is by playing 16-bit wavs or aiff's and using Cybersound 14-bit calibration (comes with the play16 archive).
If you use a double scan screen mode then you can play back 16-bit stereo audio at 14-bit 44.1 KHz. If you're not using a double scan mode (hardly anyone does), then it's best to convert your wavs/aiff from 16-bit 44100 Hz to 16-bit 28836 Hz (save as wav or aiff) for absolute best fidelity under PAL (28837 Hz for NTSC).
