If possible, I would first attempt to watch your slingbox from another computer as a test. If there is no sound from that computer connection also, then I would suspect a problem at the SB location -- perhaps the cables for the sound became disconnected somehow? Or is there a mute button on your SB virtual remote that has somehow become activated?
What I understand from your post, is that you're having problems with the Slingbox audio.
In this case, you can check the following links:
Now, if the issue is still the same, then try connecting a different Audio and Video source just to troubleshoot, run the Setup Assistant and test the connection again, all of this, to discard the current one as a potential issue.
Besides that to contact the Technical Support Department, you can use the following link