Would there be a lot of point to this feature, you already have YouTube and dozens of music players available. If you don't want a music program to hog your resources, use something tiny like foobar2000.
Windows Vista, 7 and above offers a volume mixer so you can adjust the volume for every running application: http://windows.microsoft.com/en-gb/windows7/adjust-the-sound-level-on-your-computer
Yes. You can mute specific applications or adjust the volume for example Google and 9dragons. Right-hand click volume, click "Volume Mixer" and adjust the levels of the applications until you're happy.
i was hoping to add my own grind mix but i see some dont like this idea y ru youtube wile playing 9ds it lags te game kinda dumb y i asked for tis option