I'm having the same problem as you are. I thought it may be an issue with root certificates not being properly installed in the kitchen. Do apps even access that? I'm looking at HTC Privileged Root Cert specifically because HTC Audio Manager has the same error as YouTube.
Anyway, I made a post about this same problem.. see that thread here:
http://forum.ppcgeeks.com/showthread.php?p=583506 (includes nosound VGA YouTube cab)
As well as my post near the bottom of the page in the 20931 build thread:
http://forum.ppcgeeks.com/showthread...=46526&page=12
BTW Hibby's rom has working YouTube just like the stock shipped Sprint rom.