I'm having some trouble getting the extra levels to work; I moved the files over to the game directory and tried to launch the levels through setup, but when I try to save and launch the game I get a message in the DOS box saying "SD_Startup: Playback failed, possibly due to an invalid or conflicting DMA channel.
Make sure the 16-bit DMA channel is correct."
It then brings me to the sound setup and even without changing any settings there and exiting right away, the game loses sound fx.
That's not such a problem as I can easily re-install the game, but I can't figure out what I'm doing wrong.

Probably you have the wrong value for the 16bit DMA channel. I think there are only 3 possible values for it. 0, 1 and 2. One of them is the right one, the one that dosbox needs.

dosbox it's highdma channel is 5 as far as I know.

Ah, sorry then, thought there were only 3 possible values.