Prius Model III, Volume controls are frozen

In my Prius III, beginning last month, I experienced periodic degradation of the stereo radio sound. After driving for about 15 minutes, the radio sound would cut out altogether and the volume knobs no longer were able to raise or lower the volume. Gradually, the time it took for this to happen has decreased, so now I have about 5 minutes before I lose sound and my ability to alter the volume. It is now stuck at "16" and there is no sound at all. Is this an amp?

Oh...it is the JBL sound system...