The AVR-E300 doesn't have an audio output. It's a very basic AV receiver built down to a price.
Even if you has a better quality receiver, you still would only be able to port sound from analogue sources (ones connected via red & white RCA/phono) to the Sonos Connect for distribution around the rest of your home. AV receivers don't decode audio from HDMI sources to analogue stereo.
If you want to port sound, hook up a source direct to the Sonos Connect's audio input. Don't try going via the receiver.
Answers & Comments
The AVR-E300 doesn't have an audio output. It's a very basic AV receiver built down to a price.
Even if you has a better quality receiver, you still would only be able to port sound from analogue sources (ones connected via red & white RCA/phono) to the Sonos Connect for distribution around the rest of your home. AV receivers don't decode audio from HDMI sources to analogue stereo.
If you want to port sound, hook up a source direct to the Sonos Connect's audio input. Don't try going via the receiver.