Hi @mocher72
Welcome to the Sonos Community!
The Arc HDMI connection must be connected to a TV, normally, as source devices (like your PC) put the audio on different wires in the HDMI cable - the Arc needs the audio to be placed on the HDMI-ARC/eARC channel.
Most projectors do not support HDMI-ARC, so you may need to get a device to separate the audio from the HDMI connection before it reaches the projector and hand it to the Arc over an eARC connection (eARC has a higher bandwidth than HDMI-ARC for Atmos sound).
I recommend you carefully check the specifications of your projector. If you’re unsure, let the community know the make and model of your projector - I’m sure they’ll assist.
Check out the HDFury Arcana. It extracts audio from a normal HDMI feed and offers an eARC port for Arc to connect to.