Because it is almost always annoying. 

>You probably recognize this situation: You’re surfing the web looking for inspiration, you click on some links, monitor your Twitter feed, and open pages in the background for later review. Suddenly your computer starts to scream!

>It’s some rock song, very loud and unrecognizable because of it’s way-too-low bit rate. You don’t know where it’s coming from but you want it to stop immediately so you close all browser tabs and it’s quiet again.

>Almost all people I ask about background music on websites tell me they find it totally annoying. A website just isn’t a thing you would expect to make any sound. **People often listen to music while browsing the web, there’s no room for an extra layer of sound**.

>*There’s a difference between background music and interface sounds though*. I believe the right use of the right sounds can actually improve the user experience of a website. 

Source: [SOUND ON WEBSITES: A SENSITIVE SUBJECT](http://www.everydaylistening.com/articles/2009/6/15/sound-on-websites-a-sensitive-subject.html)

[![enter image description here][1]][1]

But if you need it or want it – here are a few tips on how to do it:

> Using sound on your website [isn't always a good idea](http://www.everydaylistening.com/articles/2009/6/15/sound-on-websites-a-sensitive-subject.html).
> In fact, it annoys people more often than not. This is probably caused
> by the enormous amount of web sites containing crappy sounds. The
> right use of sound though, can enhance the user experience. So if you
> want to do it anyway, here's a list of 10 tips to make the experience
> as pleasant as possible:
> 
>  - **No background music** A lot of people are listening to music while browsing the web. Don't interrupt them by playing music on your
> website. For some sites the use of music might enhance the overall
> experience, like web/sound-art and some specific sound-related web
> sites, but don't do it without warning your visitor (see tip 3). If
> you really want to give your website a continuous soundtrack, consider
> using a background atmosphere (see tip 4). 
>  - **No auto-play** If you decide to use music, don't have it start without the visitors' permission. If you ever opened an auto-playing
> MySpace page in a new browser tab for later review, you know what I'm
> talking about. And provide a mute-button: if a user doesn't like the
> sound and there's no way to turn it off, he/she will leave your
> website. 
>  - **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.    
>  - **Make sound and design match** Try to create sounds which match the design of your site as close as possible. If your design is rather
> futuristic, it would be strange to use very natural or recognizable
> instrumental sounds. In this case abstract electronic sounds would
> most probably complement the graphic design much better. 
>  - **Care about the overall user experience** Don't just add sounds to your website because you think it's cool. Ask yourself what it means
> to the user, and how it will improve the overall user experience. If
> you don't find a clear answer to these questions, don't do it. Most
> internet users still prefer web sites to be silent. Be precautious,
> don't act like you don't care.

Source: [10 TIPS FOR EFFECTIVE WEB SOUND DESIGN](http://www.everydaylistening.com/articles/2009/6/21/10-tips-for-effective-web-sound-design.html)


  [1]: https://i.sstatic.net/OeizF.jpg