FM music, OPL chip music
This page http://www.student.oulu.fi/~vtatila/console_and_pc_music.html
has an article "FM Music on the PC"
explains that there are few PC games with FM music worth listening to. That COULD be true, because FM music is from analog+digital synthetisers and does not represent music made from samples like MOD - tracker music.
That is a problem. Emulating FM music takes <1% success. It would require emulation of electrical analog circuits which takes more than current CPU's can handle.
Here is a link to a real OLP3 chip recording http://www.student.oulu.fi/~vtatila/downloads/ski_or_die.mp3 (crazy Ski or die game)
One reason why I'm including an mp3 here is that no emulation of the OPL chip sounds exactly right. notice the loss in bass which is part of the sound, not to mention the screaming, distorted, fizzy lead sound which all the software emulations fail to replicate. The percussion kind of sucks but that's how it is in most OPL music.
The music in UFO Enemy Unknown, on the other hand, has some great UFO FM sound effects (difficult to emulate with a sample based card) and guitar-like FM sounds. The tunes are also long and complex and have lots going on at once, almost prog rock in that sense.