Loss of volume after stand-by

Every time my computer goes into stand-by or sleep mode, the speakers won't
work once it come sback to life. They are not muted or turned off, and the
media player/Internet radio station stills seems to be playing, but no sound
comes out! The only way to resolve this seems to be to reboot, which is
frustrating to say the least if I am running other programs. Why is this
happening? How do I fix it?