Erm... do you still have the 3.6.7 MVPs somewhere? If you do, you could create a directory in \games\freespace2 called "disabled" and move your 3.6.8 beta or delta or whatever VPs there. Then extract the 3.6.7 MVPs (+ patches too, if you've got them) into whatever directory you're using... hopefully \mediavps . See http://www.hard-light.net/wiki/index.php/Installing_mods_for_fs2_open
Just remember that you need to use build 12/22/05 or earlier (3.6.7 build should work fine), and when you switch back to 3.6.8 VPs, you need build 12/29/05.
On second thought... were you using 12/29/05? If not, that could be your problem.
If you want to use 3.6.8 VPs, make sure you've got the latest version of mv_core and that you use 12/29/05 or later
If you want to use 3.6.7 VPs, use 12/22/05 or earlier
A good idea, if you have the 3.6.8 VPs
still in archive format somewhere:
erase mv_* from your directory you're using
download the latest mv_core 3.6.8
download the latest CVS build (12/29/05, I think http://www.hard-light.net/forums/index.php/topic,34852.msg721495.html#msg721495
put the CVS build in FS2 main dir and the VPs in \mediavps or the dir that you choose.
Setup mod.ini or otherwise as you choose (see wiki link earlier)
BTW I'm using 3.6.8 w/ CVS 12/29/05 and no sound problems.
If this doesn't work, check your FS dir, FS \data dir (& subdirs) and make sure there's nothing there besides the retail VPs (move any mods to \disabled until you get this sorted out)
The only other problem I can think of is a corrupt download... to check this, download 7-zip from http://www.7-zip.org/
(freeware, no spyware in it) After setting it up, open 7-zip, under Tools > Options, click "Plugins" tab, then "Options" button. Check all except the last two entries and hit OK twice to close. Now, find your archive in Windows Explorer, select them all, right-click on one of them and hit 7-zip > Test Archive. It will check the CRC of your archives and come up with a no errors box if all is well (the box might pop up behind explorer, so minimize everything else if you don't see anything).
Hope this helps.
EDIT: Of course, check your sound settings under the Launcher... only use EAX or Aureal A3D if your card supports it. If you don't know, just use DirectSound
Also, check to see if you're using latest openal ... http://scp.indiegames.us/download.php?view.122