Using the Gnome interface, its pretty easy to adjust the volume of running audio based applications i.e. Go to sound Preferences >> Applications Tab. When using the Command Line interface (CLI) on the other hand, Its not that straight forward.

At the command prompt, run the following command:
pactl list | grep -A16 ‘Sink Input’ | more

The above command will give you details of your running streams or in ‘pulseaudio speak’ Sink Inputs (go figure). Take note of the Index number of the Sink Input you wish to control.

To control the volume of your application:
To mute: pactl set-sink-input-mute [sink Index no.] 1
To unmute: pactl set-sink-input-mute [sink Index no.] 0
To set Volume: pactl set-sink-input-volume [sink Index no.] 65535 (0-65535)