Your AVR is responding to the handshake from the monitor. There should be some setting in the AVR menu to fix that by not sending audio to the monitor or defeating some feature.
PCM audio via HDMI
I have my cable and Blu Ray player hooked up to digital audio and HDMI cables running into my Marantz SR5007 A/V amp. When the monitor turns on the 5.1 lights go off on the speaker array and PCM audio shows as the only available audio mode. When I turn the monitor off the audio kicks back up to a far more robust 5.1 audio output. Why is my audio being compromised and can I defeat the audio on the HDMI to avoid this problem?