Everyday Listening has a great list of things to bear in mind when working with sound for websites. Here’s a selection:
• Unobtrusive interface sounds
Adding sounds to user-interface items can absolutely enhance the usability of your site. Try to keep these sounds unobtrusive in a way that a user can still hear them, even while listening to music, but without being disturbing while browsing in a quiet environment.
• Keep sounds short
A good way to prevent sounds from becoming annoying is by keeping them short. Imagine navigating a website with a second-long sound playing for every link your mouse cursor touches, that wouldn’t be a pleasant experience, would it?
• Use the right frequencies
The human ear is most sensitive to sounds between 1kHz and 5kHz, so if you want people to hear the sounds, even on crappy laptop speakers set to low volumes, focus on this frequency range.
• Add extra functionality
Do it right or don’t do it at all. If you decide to use sound on your website you can give it extra functionality, more than just providing auditive feedback. Variations in timbre and pitch of the sounds can be used to tell the user more about the menu or the page he/she is browsing.
Read the full list here.