SoX (also known as Sound eXchange) is a commandline tool which translates sound files between different file formats, and optionally applies various sound effects.
SoX is intended as the Swiss Army knife of sound processing tools. It doesn't do anything very well, but sooner or later it comes in very handy.
The no-ixemul MorphOS version was build against libmad and libmp3lame and comes therefore with mp3 support. Following formats are supported:
aiff, auto, avr, cdr, cvs, dat, vms, gsm, hcom, maud, mp3, nul, prc, raw, sb, sf, sl, smp, sndt, sph, 8svx, sw, txw, voc, wav, wve and a few more.
Download it from here.