This just happened to me today!
Just like what happened to dragnfly. I was listening to my radio. I went to turn it up when I noticed that I couldn't adjust the volume on the lexus dashboard, either up or down. I shut off the sound system and turned it back on, but all sound had disappeared.
Everything else seems to work perfectly. On the radio, I can change from FM to AM and switch stations. I can see that the changes are occurring on the info screen. On my CD player, discs change, and tracks play, complete with elapsed times. When I put a cassette in the tape moves as usual. But I get no sound at all from any audio source. I can't hear anything via the lexus radio, tape deck, or cd changer even if volume is maxed.
None of the lower sound controls work. I can push the left button to turn the system on and off but turning the volume knob does nothing. I can push the right button to toggle through the sound options like treble and bass, but they all default to zero, and turning the knob does not change them. I can push the mute button but nothing happens and I don't see the little "mute" displayed on the info screen.
I changed both audio fuses, thinking one must have blown. But the fuses looked fine and the system still has no sound. I thought it might be the power amplifier, so hopefully getting that component repaired or replaced will do the trick. I just have to figure out how to get to it.