They are not ripped directly from the game i think.
2 years ago or so I've read that games from hexen, and probably any other doom engine based game used snd files for ingame tunes.
A converter had been written to convert the snd files to midi, so the original opl data probably goes lost.
Then again, the snd files are probably just the files that are used when selecting general midi or roland in the setup of these games.
Maybe snd files are a special format that both can be used for midi and opl (for example, when midi, instrument is played, when opl, the instrument in the opl bank is played)
I'm just guessing here, but anyhow, I strongly doubt that the midi files that you can find all over the internet are representative for the opl ingame music of doom engine games...midi(snd) and opl soundfiles in doomgames are possibly not the same files, and if even if they are, keep in mind that the music has been converted to another format which could mean loss of essential data for correct playback with adplay or so.