Recently the sound from the standard (non-Bose) audio system in my 2004 Infiniti G35 sedan stopped working.

The sound system worked fine for a decade, but started having problems in the past month. For a little while the sound would come and go at random times, but now the sound almost never comes on at all. When the sound does come on, it begins with a popping noise; then I can hear it, but at unpredictable volumes (sometimes loud, sometimes soft). And what's worse, the actual volume no longer corresponds to the volume I select - it just plays at whatever volume it came on at this time.

All the controls on the radio, climate control, steering wheel, and display functions seem to work. I can change stations and select a different disc. I can also see the volume level change on the display, although it doesn't seem to affect the sound output at all (unless I turn it entirely off).

Because the audio controls and the climate control system work fine, I'm guessing I don't have a problem with the integrated control board. Rather, it seems likely there's some problem with the radio unit itself. (My best guess is an un-soldered or blown volume resistor or capacitor.)

Does anyone have any insight into this problem? Any thoughts on how I should go about fixing it?