dbears, welcome to the Interact Forum!
c) I think that much more information is needed before the video sound problem could be solved here.
For starters:
What format are your videos? Both video format and audio format if possible. If you don't know that you could find out the file extension and tell how/where from you got those videos. It helps a little.
I suppose that you are talking about video files on your HD. (Or are they something else: net stream, DVD, TV broadcast by TV card, ...?)
Do other programs play those video files with sound? E.g. Windows Media Player, RealPlayer, QuickTime.
Is the audio working with any other video files? (In MC or in other programs.)
What is the brand and model of your USB audio device? What drivers are used (also version)? How do you connect that to your stereo - e.g. analog to RCA connectors or is it a digital connection (coaxial RCA or optical)?
b) The same goes here. More information, please. Is the problem MC specific? What do you see on your TV? Can you show videos on TV with other programs? You should try various GeForce driver settings. There are probably settings for S-video/composite, secondary display, theater mode, etc. I have only ATI cards, so I can't help much here.
You should also post your system information. It's in the help menu. I'm not sure if that helps in this case, but anyway. Like this:
Media Center Registered 10.0.119 -- C:\Soft\MJB\
Microsoft Windows XP 5.1 Service Pack 1 (Build 2600)
Intel Pentium 4 2857 MHz MMX / Memory: Total - 1048 MB, Free - 628 MB
Internet Explorer: 6.0.2800.1106 / ComCtl32.dll: 5.82 (xpsp1.020828-1920) / Shlwapi.dll: 6.00.2800.1400 / Shell32.dll: 6.00.2800.1233 (xpsp2.030604-1804) / wnaspi32.dll: Internal ASPI Layer
Ripping / Drive G: HL-DT-STRW/DVD GCC-4480B Mode:ModeSecure Type:Auto Speed:Max
Digital playback: Yes / Use YADB: Yes / Get cover art: Yes / Calc replay gain: Yes / Copy volume: 32767
Eject after ripping: No / Play sound after ripping: No
Burning / Drive G: HL-DT-ST RW/DVD GCC-4480B Addr: 1:0:0 Speed:12 MaxSpeed:0 BurnProof:Yes
Test mode: No / Eject after writing: Yes / Direct decoding: No / Write CD-Text: Yes
Use playback settings: No / Normalization: None