Is there an app that can do this?

Not an app, but Firefox’s FlashBlock will prevent any flash objects from launching until you give the go-ahead. It’s useful enough that I expect that someone has made an app like it.
There are, as yet, no apps that can read your mind; you’d have to select a criterion by which they will operate. I figure that you can either block by type, or block by point-of-origin.

Reading my mind is obviously not a requirement. I don’t use firefox, but I will look for a flash blocker.. hmmm.. hopefully it works. =)

You can’t choose an audio stream from a source like that, its all or nothing.

@XOIIO Isn’t there some lower level connection blocking I could do? There’s got to be a way to stop it. It’s unbelievably annoying.

@Meego If but only I wanted to watch the video silently. XD

I find judicious use of AdBlock to work just fine.

This question would be much easier to answer if you tell us which browser(s) you use on what sort of device (PC, smartphone, etc) with what operating system.
The plugins proposed by @Nullo and @jerv are excellent at what they do. The one I use is NoScript which blocks all client-side scripts (and is therefore somewhat annoying if you aren’t used to it.)

There is, of course, some “lower level” connection blocking you can do, especially if you have a stateful firewall and know how to give it rules. That route does require a bunch more information though.

For the operating system, the complete browser only provides one audio source, so all audio is mixed into one stream that is sent to the sound card or mixer.
I would start with AdblockPro or an alternate ad control software and try to build rules with that.

@koanhead I also used noscript when I was using firefox. I now use google chrome and I’m trying somethind called flashblock thanks to @Nullo‘s response. I guess I will wait and see if it does the job.

I was thinking there might be an app out there that actually brings up a list of the audio streams coming in and would allow you to “allow” only the pertinent ones. I suppose not yet?

@alexlehm Thanks for the explanation. I didn’t know it was all bundled up in one source. How does that work though? Let’s say I have two windows open, both are playing a youtube video with sound.. that’s on one “channel/source”?

You could probably configure an adblock/ now opt with exceptions like megavideo.com or videobb.com

@XOIIO Hmmm, hell I’ll try it… =)