Try the command line tool:
http://aminet.net/mus/misc/cutmp3-bin-m68k.lha

copy bin/cutmp3 and bin/sh to sys:c/

open a shell window to the drawer with your mp3 file and
type something like this:

cutmp3 -i Donots_-_Das_Ende_der_Welt_ist_längst_vorbei.mp3 -a 0:37 -b 0:50 -O Ende.mp3

thats all.

greets