First there are two possible audio sources here.

1. The XRX (using audio cable provided or other), to turn the volume up on the
XRX's audio output.
On the main traffic page, press the up button and you will hear "TEST" in your headset,use up and down to set a reasonable volume.

2. The audio from the Garmin 396/496, the volume adjustment in the Garmin is different. It is found in the setup menus, from the main page press menu twice.
Scroll down to Sound page, there are two adjustments here on TIS audio there are options of On, Medium, and Loud. At the bottom of the sound page there is a main volume adjustment, left and right adjust all of the volume going out of your Garmin.

Thanks