Thanks for the replies. Yeah, the audio is coming from the red and white lines from the Gamecube’s A/V port. This is the cable they’re going into, which then goes into AV2 on the OSSC:
Even if that was going stereo to mono, I’d figure that some kind of audio would be output, but maybe not? Is there a good way to push the switch back after it’s been pushed in, as well? There doesn’t seem to be much room to get at the rear of it.