I bet it's a physical block. You might be able to get around it if you know how to crimp a network cable.
Make sure you have an RJ-11 end for your network cable to replace
Unplug your network cable
cut the end off
unwind the "twisted pairs"
Find the one with a white stripe
Peel it back, away from the others
Place all of the other cables back into the new RJ-11 prong
Crimp that end down
Plug it back in
White = RCA video. Removing that connection will allow you to circumvent any firewall setting saved in your NIC chipset that is related to video. Since your monitor and the PC are still able to communicate, your video will display and, of course, audio will not have been affected because your sound card has no firewall.
