That said, it is believed that MPEG-1 audio layers 1 & 2 and MPEG-1 video are free and clear in the US as of at least 2008. No lawyer or judge has made that determination however, and AudioMPEG Inc. claims their layer 3 (MP3) patents also cover layers 1 & 2 (although people who've implemented decoders say otherwise). No such claims have been made for MPEG-1 video.
Given that hundreds of commercial games ship with FMOD, and they say you can use MP2 without an additional license (but specifically state you need one from Thompson for MP3 which includes giving them royalty payments) I think there are potentially bigger targets than MAME if MP2 isn't 'safe' to be honest.
This is a commercial sound library, and they've dealt with the 'big players' in the audio playback market, so I think they know where things stand.
Of course, that's just as screwed up anyway, because the MP3 code is still in the binary, just not used :-p