Hi
Start by clearing the VP cache of unused files. (Tools/Options/Disk). Whenever you make an edit wait until the green bar above the thumbnails has finished moving. Try setting the preview to a lower resolution. (Tools/Options/Display...Preset)
I cant say why the soundtrack keeps playing (NCH might though) At the end of the day it may be your PC. :-( . Does the "Stopping threads" message appear when you stop VP and discard the current project?
Nat