In-Game volume mixer

I love to listen to music when I'm playing games. But I use Spotify, is there a program that works like the volume mixer and lets me only change Spotify's volume instead of constantly minimising the game? (One that over laps the game, like xfire)

Thanks In Advance

Carl Lawrence said:

Have you tried Auducity?

No. What I want is, like when I press maybe tab, something like steam or xfire comes up but it allows me to change the volume of other programs from In-Game