NS-R5101HD same as Pioneer VSX-819H-K Receiver

Looking at the block diagram in the Pioneer VSX-819-HK service manual, my suspicion would be that something may have gone bad on sub-assembly K (the INPUT-819 board). In particular, IC801 (R2A15218FP) is a big monster which performs the analog input selection, analog/digital switching, and volume adjustment functions. It has a simple control input (DATA and CLOCK inputs from the central micro-controller)... could be something like an I2C or SPI interface I suppose. If this IC has developed an internal fault, then the analog signal paths would go wonky, and the microcontroller which runs everything would probably not know about it.

If you have an o'scope, you could feed a test signal to the receiver's AUX inputs, select AUX, and then look at the signals going ito pins 75 and 76 of IC801 (AUX-In to the selector) and at the various outputs to this IC. If I'm reading the block diagram correctly, pins 54 and 55 should feed the selected input signal out to the A-to-D converter (there are on-chip level controls), while 21 and 22 drive the signal out to the "front right" and "front left" amplifier channels (there's a whole bunch of switching, volume control, and other stuff going on inside the chip in this path).

If the signal isn't getting through IC801 properly, you'd either need to replace this big IC (if it's even available separately) or replace the whole K sub-assembly board.