I think I understand your confusion. Audio Overload plays sound rips from games, not the actual ROM files like M1. (The rips contain data from the ROMs but reformatted so only the music player is emulated). For NES, these are in a format called .NSF. You can find rips for a variety of systems at http://www.zophar.net/music.html
- AO can play nearly every format there except PSF2. Also, AO can play them even from inside archives, including zip, rar, and cab.