We are not suddenly getting sound out of our stereo.
I tried switching out speakers, but still no sound, so the speakers are not the problem.
The receiver (Pioneer SX-255R) has A and B outputs, I've tried both, but neither produce sound.
I plug headphones into the receiver and I hear sound just fine.
There is no sound for am/fm or CD. (no other peripherals to test).

What else can I check?