Hmm, this is a tough issue because there really is no clear, definitive answer. I like to look at issues like this:
Technical:
If a client wants a feature (in this case sound) perform the due diligence to research if browsers can support that feature, and which ones will not. Come up with numbers to show the client. X feature is only supported in X browser and most people use "this" browser so I would / wouldn't use this feature.
Psychological:
Specifically for sound, study the psychological effects, usability of this feature as it concerns the end user. I will immediacy leave a website if a video or sound starts playing without my permission. I expect a choice, and when that choice is taken away I leave. And at the point where I leave, I'm mad, I hate any user experience where I am left out.
Client is not always right:
So most of us have heard that the client is ALWAYS right. Well to be quite honest, no no they are not ALWAYS right. It does not matter whether your selling websites, or magazines, or working on cars, you have to be there for your client. Obviously if you do good work people will refer you. However sometimes you have to stand your ground with a client. My suggestion is to make sure you do that at the beggining of a project, rather than later. I've turned down projects, or agreed to handle only certain aspects of a site design because I wouldn't be associated with certain features. For example I don't do Flash sites. Not that flash is bad I just don't do it. I give those projects to friends. And they return the favor. If you tell a client upfront that you won't / can't deliver that's a good thing. Don't say yes and then come up with an excusse later in the project, that's where the client will become frustrated with you, and if they complain they are right to do so, and you will loose business.
At the end of the day communication, deciding upfront what you will and will not do will save your lots of headaches.
And as for sound, it has it's time and place. Bands, Flashsites especially those highend national ad campaings for cell phones, or movies can get away with sound. The best option at the beginning of a project is to tell (don't ask) the client that they can have sound, and if it does auto play you will set he volume to low, and have a visable player that the user can control, meaning they can TURN IT OFF, OR LOWER THE VOLUME, these features are not negotiable. If they have a hard time with that, then walk away from the project because they will have a hard time with anything. And don't be afraid to turn down work. For every 3 sites I work on I turn down one.
I recently took on a project that requires sound. I'm kinda in a pickle with my client (he's not mad) but he told me he wanted sound and I offered to use a player, and give control to the end user. He was ok with that. Recently, after checking out the sound player feature he says "No, I wanted a sound to play when you rollover the navigation. The pickle is that he never said that, and I've stood my ground with him about adding that feature. So he's a little upset with me, but we are working it out. He's mostly upset that I want to charge for the extra feature, and I'm not budging. It will all work it. Just an example.
Sorry for the long reply, Good luck!