Sue me, but I'm gonna out and say it: sound management under Linux suxx big time and it has done so for the longest time. It's laden with incomprehensible jargon and the most intricate jumble of competing technologies you can think of. Bottom line: lots of times, you have trouble getting audio out of your system.
My problem: FC13 with kde (but other WMs also, so KDE does not really enter into it) does occasionally:
1) switch unrequested from "Internal analog stereo" to HDMI Manhattan whatever
2) Forget entirely the "Internal analog stereo" device (shows up greyed out in multimedia)
Guess what? the "Internal analog stereo" device is the one outputting audio to my headphiones and loudspeakers. There are two hard things here: understanding (better: get a feel for) what's going on and fixing it.
I have (as of now) no general guide but I have found these commands to be of great help in finding out what the problem may be:
yum install pulseaudio-utils
These - paman especially - also help in fixing the "switch device" problem, while:
Appear to fix the forgotten device problem.
All for now...