How to decrease and Increase Sound of chosen App? When I was playing Counter Strike: Global Offensive, I noticed that sometimes, I want to listen to music when I am dead in-game, waiting for the next round without the gun sounds of the game mixing up with the music that I am listening to and then switching back to the game, decreasing the volume of the song that I have been listening to. 

On Windows, this is possible. Is it possible on Mac as well? If yes, how?
OS X doesn't have a global manager for volumes of specific apps unlike Windows. Rather, you can change the volume of apps inside the app themselves.

For example, iTunes has a volume slider inside the app that lets you change the volume of iTunes, and whilst I'm not familiar with  the options menu of CS:GO, I presume it has a slider for volume. Use these controls for changing the volume and script them if necessary using AppleScript or equivalent.