> I have to install the decoder?

No, you can set AmigaAMP to use the mpega.library, which comes with MorphOS, instead of the dedicated decoder. There may be a small workaround required to make AmigaAMP run on MorphOS flawlessly:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=7650&forum=9&start=4