I have the 6000 NEX installed in a 2002 Audi A4. Intermittently, I will not get any sound from the speakers. It doesn't matter what source is selected, HD radio, Bluetooth, video on SD card, etc.  I can still hear the beeps from the headunit when pressing buttons, but nothing from the speakers. It doesn't do it everytime, and sometimes I get no sound from initial start of the vehicle, and sometimes as I am driving, it will cut out, come back on, and do that a few times before it stops working. 

I had RF filters installed because of popping when turning off the vehicle, so I took them off thinking they might be the problem. No luck. I am not sure what it could be, since I don't get any sound at all from any speaker. I double checked wiring, and it is all right and connections are good.

Today, when I pulled off the RF filters to see if they were the problem, I started the car with the headunit pulled out, and I was getting sound intermittently, and I noticed the fan on the back wasn't running and the heatsink around the fan felt hot. Not hot enough to burn anything, but I kinda figured the fan should run all the time, or atleast when it was hot (and since the heatsink was hot, I figured it should be on at that time.) So does anyone know about the fan, and if that might be the problem?

I have had this problem only with CarPlay. Sometimes when I turn the system on, I will not get any sound. If I unplug the phone and plug it back in the sound comes back.

I think it already happened after a phone call also.

I have a Google Nexus 4, so no carplay here. I have tried turning off the car and turning back on, and it doesn't work. I have also erased the bluetooth devices from the radio and phone and then reconnecting, I have removed the buttons on the front of the radio to turn off the power while driving and nothing seems to work other than turning off the car and waiting.

most modern amplifiers, quite likely including the built-in amplifier, have thermal overload protection.  That means if it gets too hot it will automatically shut down at or near a predetermined, preset temperature, and automatically reset as it cools.  The temperature sensing device is typically embedded, attached, or held in place against the heat sink.

Fan not running while heat sink is hot is a good sign of either a fan failure or it's control circuit. Audio cutting out is likely a result of the thermal overload protection.

I have the same problem sometimes only when using CarPlay. I'll turn the unit on but get no audio. I pop the phone out, plug it back in, and it works. Drives me crazy.