Im interested in buying a NL mono cart. Does anyone know if it works with the Game Boy Player?

Since it doesn't work well with GBA, the probable answer is no.

probably would work with a super gameboy though

I asked Oliver awhile ago about this and the answer is that it won't fully work.

I asked if there was a direct audio out mod like on other Non-DMGs, but I never got an answer.  I know a Super Gameboy can be prosound modded, but don't know about the audio out pin mod.