If you've got a Jensen VM9726 (standard with at least some of the 2015 Soleras), the system has a whole variety of ways to set the volume, including setting it based on the source of the sound (radio, navigation, blue tooth, etc.) I'd strongly suggest looking at the manuals (both the basic VM9726 manual and the navigation-specific manual) to figure out the various menu options to check and set volume.
The Jensen VM9726BT web site is here:
Jensen : Mobile Multimedia : VM9726BT
That page has a section "Support/Manuals" with links to both the basic manual and the navigation manual.