The pre-announcement of tepples' volume test set inspired me to write something as well (fortunately, we took different approaches so the two complement each other). The tests generally play tones with the channel under test, then manually generate the exact opposite waveform using the DMC DAC, so that they cancel out to silence if everything is correct. Included is ca65 source and recordings of them on a NES.
mixer_test2.zip
mixer_test2.zip