When you built the cues, were you using an external sound card or some other device? Wondering if the output patch may be set to a missing device.
On the cue you are trying to play, in the Device & Level tab, do you see something like this:
Also - the levels should be more or less as shown here.
Then check your patch assignment for crosspoint levels on the Built In output patch - go to the lower right of the Qlab window and click on the gear, then select Audio in the right-hand list you see.
Check your crosspoint levels to be set at "0" and your input levels to be set at "0" as shown here:
Finally - make sure your soundfile is good - I was just playing a couple of files in order to set up these visuals, and one I picked (believe it or not in the context of this discussion) turned out to be silent (a recording of silence I'd made to use for timing in an odd sequence)!
Dave Tosti-Lane