Sound is rather complex in Linux, since there are several
programs to handle audio, and more programs that allow
multiple programs to use audio at the same time.
Most likely you have a program, such as firefox or flash,
that can take over the audio channel and not release it
for other programs. If you discover that sound stops
working, try closing a program at a time until you find
which one is causing the problem.
You may need to get assistance from someone using the
same distro, desktop and sound daemon.