Very High Quality plays only 3 of the 4 tracker channels. This modification on the song seems to cost some extra waiting time during every level-loading screen.

It does not render the song mono; still, each of the 3 channels only comes either from the left or right speaker. E.g., if you capture only one speaker afterwards, the sound will sound like even more tracker channels were cut.

Disabling music altogether in the setup tool makes for extremely fast level-loading screens.

Very High and Ultra High also change how sfx sound in Dosbox. E.g., the pitched sound effect on shooting a 15-ammo container sounds nicer on Very High, and a tad jagged on Ultra High.

Haven't toyed with the other quality settings besides the highest two. Lovely topic, good luck on the research!

-- Simon