Trying to add about 24MB big track, and it just does not work. Sounds is turning off automatically randomly even after render, so its not problem only in preview, and as you can see it completely turn off sometimes.
I've uploaded on Youtube, it starts at 00:20
Any chance you are using an unlicensed audio effect?
I have on inadvertently enabled a track or master mute automation resulting in loss of sound at certain points, which was puzzling at the time.
Some obscure audio formats do not play nice with Vegas. Check the files with MediaInfo for a possible unsupported codec.
Simply making a new project and copy & paste everything there helped.