Check the track header (at the extreme left end of the track) to make certain the audio track isn't muted (the round icon with a line through it.) Also, try clicking on the "solo" icon -- it looks like an exclamation point .
to be codec-related or something similar. I say that because I just realized that my winamp is playing sound either. But the bsplayer and the wm player are playing just fine.
If you have just installed then all the factory defaults should be allowing you to be good to go.
Have you tried resetting the factory defaults? If you do any User prefs will be removed. If you have done some settings then make a notepad or pencil or screen grab of the various settings ready fore you to re-enter.
I'd be inclined to do a factory reset just to remove any install issue and to get everything back to how it should be. If it still persists then you need to look further.
Factory reset hold down CNTRL+SHIFT while starting up Vegas.
Make sure you have something sensible selected in Options|Preferences|Audio Device|Audio device type:. A setting of "Windows Classic Wave Driver" should give you normal audio output and is probably the best choice to start with.