For those who haven't looked at the back of the Yamaha...
AV2 & AV3 are composite video + analog audio (RCA) inputs.
Audio 1 is a optical digital audio input.
Audio 2 is a coaxial digital audio input.
AV1 is a composite video input with a coaxial digital audio input.
Aux is a 1/8" mini stereo connection on the front of the receiver, so that's why it wouldn't be as high of quality (potentially) as the RCA connections on the back.
So, it would be great to have
@First tell us what source he is using that is plugged into the Yamaha on AV2 and to ensure that he has properly tested the RCA cables, as described above.
My first reaction, always, is that a RCA cable is bad.
Test tones work well to both speakers. So, your speaker connections are fine.
No audio from one channel. But, you must test the RCA cables to ensure that both the cables are good.
You also must test from the source.
That is, if the red cable is good to BOTH channels when you switch it, and the white cable doesn't work to either channel, you must put the white cable where the red cable was at the source and be sure the source is properly putting out audio on both the white and the red cables.
If you are using a source which has a 1/8" stereo output, it could be the breakout cable you are using. Many of them are low quality. If you are using an older player with RCA connections, one of the output connections could be bad.
BUT: Most often, with what you are describing, it is just one of the cables, or an adapter you are using which has gone bad.
Here's a good page with nice big photos of the Yammie...
https://www.crutchfield.com/S-vjHwkdHhD1N/p_022RXV385/Yamaha-RX-V385.html