Why does Ubuntu sometimes make sound inaccessible?

Sometimes, the following happens:

   (a) I am happily listening to a file through VLC or watching some video on YouTube (via Firefox)

   (b) Then I finish doing (a) above and use the 'suspend' option in Ubuntu to save power.

   (c) Next time I wake Ubuntu up, sound is unavailable. I check everything I can think of
        but nowhere is there any indication that either Ubuntu or the application I am
        using (VLC or Firefox) thinks the sound has been muted.

   (d) Even when I use the Sound tool in System Settings to check the line out, I get
         no sound.

   (e) The only way I can get sound back is by shutting down the system and starting it
         up again, which is very annoying because I lose data on what I was trying to do.

Can anybody tell me how to get sound back without having to shut down and start the
 system again? Note that, as I said in (c) above, the Ubuntu sound tool in System Settings
 thinks that nothing is muted and the same is true for VLC and/or Firefox.

Thanks.