Sorry, I am still not clear, because audio signal is not specific enough in this case because there are the signals coming out from your source (PC, mobile devices etc.),input to a streaming devices, routers etc., from there output to your AVR via HDMI, analog etc., or from your source direct to your AVR via the Ethernet and then the AVR handle the signal, amplify it and output it to the speaker. Any part of the signal path could be what you refer to as the audio signal.
So, again, did you mean it is the "input signal" to the AVR, that is, NOT the AVR's output to the speaker? If so, how do you know that for sure?
This is important to know for sure because if it is the sound coming out of the speakers that got cut out, then you cannot rule out the AVR. If somehow you have already determined that it is the signal input to the AVR, that you referred to as the "audio signal", then the issue is upstream of the AVR. In that case it would mean most unlikely (though not totally impossible) the AVR is not the issue and we can focus on issues related to the upstream devices such as the Airport devices you mentioned, and everything else upstream.
Another stupid question I have is, could you locate that "switching relay thing.."? Did you hear the switching click sound or you are just guessing there is a relay switching?
This is an interesting issue you are having, so thank you for sharing, and please do report back when you have the issue nailed down and fixed.