Turn Off Unwanted Sound Ads?

In the last 2 or 3 weeks, I have had problems with unwanted sound ads popping up unexpectedly while using my web browser. Often, these ads come at very inconvenient times and interfere with whatever I am trying to listen to at the time. I don’t want to be bothered with these ads and I consider them a form of spam.

How do I find the processes that are running these voice ads? How do I turn them off? How do I prevent them from starting in the first place?

These ads are running both on my laptop and desktop PCs. Here is some configuration information.
Browser: Firefox 15.0.1
Laptop OS: MS Windows Vista x64 SP2
Desktop OS: MS Windows 7 x64