What are you using to judge the sound output? and how have you set the sound system up? and what are you running it on?
AMPlifier pre OS3.9BB2 was very naff - especially on mp3s. The latest version with the 1.4 mp3 codec is much better - very little aliasing of high frequency signals.
Are you using the right AHI driver for your version of WinUAE? Tweaking AHI inside the emulation took me a while, and it was by guesswork, listen to result, change, reboot amiga and listen again.
And is your PC capable? Mine's an 1800+ athlon using SDR RAM, and it's happy except when I alt-tab between amiga and pc (addressed in the latest version 0.8.22r4 if I read the bugfixes right - not at home right now and separated from it)
If I can help, contact me.
Alphonsus.