Sounds should be consistent with the content and tone of the website. You probably should not contain music unless you are working on a music site.
Rationale: Sounds should add to the content of the website rather than detract from it.
Unless the sound is a primary downloadable content, sounds should only play for a short duration. Do not use endlessly looping sounds or background music. Provide the user with a way to terminate long music files.
Rationale: Long playing background sounds are annoying. Background music gets old fast and draws attention away from the real content of the webpage. Short music clips download faster.
Do not use sounds that are inappropriate, offensive, or cutsey.\
Rationale: Such use is unprofessional, and detrimental to the objectives of almost all websites.
Do not use stolen sound files.
Rationale: Almost all sound files on the Internet are copyrighted, and their unauthorized use is illegal. Such usage is unprofessional, and not worth the risk in terms of legal and branding problems.