I read somewhere that HDMI only provides 2-channel audio from a Blu-ray player or other source device to an A/V receiver. This implies that the coax digital output from the player must be used for 5.1-channel audio to the A/V receiver. Is this true? If so, does the same apply to the digital out of a satellite receiver?
What you read is absolutely, unequivocally not true. HDMI conveys multichannel audio from any source device that provides it to an A/V receiver. In fact, HDMI is the only way to digitally send the multichannel lossless formats of Dolby TrueHD and DTS-HD Master Audio from a Blu-ray player to the AVR, and these formats offer higher quality than anything that can be sent via coax or optical.
If you have an A/V question, please send it to firstname.lastname@example.org.