I probably ought to post this in Science and Technology but I thought it would reach a larger audience in this forum.
Over the years there has been debate about whether it is better to leave your computer switched on all the time or whether to switch it off at night or if it is to be unused for any length of time.
Now we have the definitive answer, there is a problem if you leave it switched on, but this is limited to computers using XP.
Check out this page and ensure your speakers are switched on.
I will never leave my computer ticking over again.