Chrome quietens other audio sources

Ok, something odd has started happening. Whenever any noise is made in Chrome (be it Youtube or the Facebook chat "pop") it makes other audio sources (such as my media player) much quieter.

Here is a picture of the Windows audio mixer when this happens:
http://img151.imageshack.us/img151/9823/audiot.png
As you can see, there is a ghost slider in the position of where Mediamoney volume was and the real slider at the volume that it's been changed to.

Some notes:
Running Windows 7 Ultimate.
All Chrome extensions have been turned off.
This doesn't happen with other browsers.
When multiple media players are playing it quietens them all.
When Chrome stops making noise (i.e. close youtube tab) it doesn't raise the volume back.

I haven't noticed this behaviour before in Chrome or any other program.

Any ideas to stop this happening?