You can change the volume of each connection individually. Hover your mouse over the connection you want to change the volume for and a volume scroll bar will be displayed (Figure 1):
Figure 1 Volume slider
Adjust the slider by moving left or right, or utilizing the scroll wheel on your mouse to change the volume accordingly.
You can mute the connection by bringing the slider all the way to the left - subsequently a mute icon will be displayed (Figure 2):
Figure 2 Mute
When a connection is muted, you will continue to see the red signaling on the connection if it is in use by your counterparties, but you will no longer hear any audio on the connection.