[popcornmix]

Multichannel PCM is only available through HDMI. Not sure your receiver supports that.
If not you'll need to get to optical somehow. Possibly your TV/projector can output that.
You may need an optical splitter, or an I2S sound card.

However with optical you can't get multichannel PCM. The Pi can transcode to 5.1. AC3 which can be transmitted over optical.