Oh, ok.
So you've got your TV connected to your home network and you're streaming content off of your PC via your network. Got it
Yes, to be perfectly honest, I don't know how the ARC system in your TV's HDMI input will handle that sort of thing. Like I said, the ARC system was really meant as a means of just getting the internal antenna tuner's audio to the receiver without needing an additional connection.
My suggestion would be to just run either an optical digital or a stereo analogue connection from your TV to your receiver if you want to keep your connections the same. Otherwise, the best way I know to make sure everything works is to plug everything directly into the receiver, as I mentioned. Run a direct audio connection (HDMI, optical or analogue) from the PC to your receiver and that's sure to work.
If you only want to keep your PC on the network, one other potential solution is to get a streaming set top box like a Popcorn hour or a Boxee Box. Those devices can connect to your PC with DLNA and pull content off of the internet or from your LAN. Then you plug those set top devices directly into your receiver with HDMI.
If your TV has DLNA support though, you shouldn't technically need any additional devices. I'm not sure if there's anything in your TV's manual that might help. Could be worth a look though. And yeah, if ARC isn't working via HDMI, then using the optical output on your TV to your receiver ought to do the trick. Many TVs are limited to stereo output only though, so that might be a problem if you're trying to get surround sound.
Sorry I can't be of more help
