This is an AV unit, correct?
1. Pull all accessories off the set.
Switch in and out of 2-channel and surround modes
2. If that doesn't work pull the speakers and try.
3. If 1 or 2 works check connections and speaker impedance at the speaker terminals.
4. What kind of volume are you talking about? Leave the volume completely down while checking and bring up slowly.
5. If all of this fails it's time to go in and find any blown fuses or bad components.
a. Note the location of any blown fuse(s). Replace. If you do have a blown fuse it's best to figure out what's blowing it first and not just blow fuse after fuse.
b. Note any burnt spots on the board or bad looking components. Obvious sign to discontinue and repair.
c. RELAYS - At least one is bound to be opening to protect the set...find the reason. The relay is not usually a problem.
d. OUTPUTS - Are any bad?
I remember the Sonys I had were neatly marked on the board with lines to denote the subdivisions, maybe Yamaha is that tidy as well. You will have fusing at each section. If you switch out of surround mode into 2-channel and it comes up, you have something wrong in the surround amps.
Another thing I remember well, in the case of Sonys is that I was told by a Sony employee to NEVER operate the B (Hafler) pair and surround at the same time as the 2-channel (stereo section) impedance will be pulled too low and you will lose one or the other. And I can back that up with the number of Sony AV receivers I've some across that are busted that way. Maybe the power supplies are too puny to handle it. I'm not a stereo doctor and I don't play one on TV. Dr. Oz is in it for Oprah's chicks.