Hi Sabbate, nice to see you here!
The last AmigaAmp working flawless on MorphOS is 2.24. If you go on AmigaAMP website you will find a link for download it but it's not valid since it will download the 2.25 version which have great problems on MorphOS (equalizer doesnt work correctly, window playlist doesn't work, it's not possible to change modules position as you like). So download 2.24 here:
http://www.amigaamp.de/Binaries/AmigaAMP224.lha
The website is the same but that link is not visible. As Andreas wrote you can use the mpega.library but this will result in a not functional equalizer ( a warning message will appear if you will try to power on) though mp3 will be played. I set the engine to "auto" and this work without problems with equalizer working flawless.