Familiarize yourself with adding actions to buttons. 

The command to change the volume level of one or more audio sources is vol

The parameters are;

• val=<new volume from 0.0 to 1.0>
• delay=<time in millisecs> (optional)
• <audio source>
• <audio source> (optional)
• <audio source> (optional)

The optional delay specifies a time (in milliseconds) to wait before running the command.

Example - change the volume of the Aquarium and Music audio sources to 0.9