The mouth movements for the b and the m are almost identical and without amazing audio quality it can be very difficult to tell whether the sound was a oral or nasal. The Klingon b and m are pretty much identical to the English "b" and "m", so if it sounds like they are switched to you, it is an audio problem and not some weird pronunciation.
It's even correct to pronounce b as mb, or b and m the same way in Klingon. (Speakers who do the latter use extra words to disambiguate, the way some Americans will say "ink pen" and "stick pin" because they pronounce pen and pin the same). We're NOT using one of those dialects here, but you can feel confident that the difficulty distinguishing is real, and not you.
I listen to the French course and the sounds are so clean, because the "big" languages have synthetic voices on Duo, with no actual lips, saliva or need to breathe, while I'm sitting here on the couch, trying to say Qatlh without blowing or spitting on the mic, redlining the Q or sounding surprised, angry, or singsongy.
Thank you so much for beta testing. Please keep reporting anything that doesn't sound right, and the way you did it here is excellent. When people just click on the "audio doesn't sound right" option, it's hard to know what it wrong.