This is from the Z1 manual, check chapter 10 page 91 "Modifying the General Settings for Navigation Functions" It might be different in the Z2 manual.
Entering the Settings Menu
1 Press the MENU button to display the
navigation menu.
2 Touch [settings] to display the Settings
menu.
3 Change the setting.
Adjusting each item ➞ Page 91 to 98
❒ This section describes the details of the
setting for each item. Settings marked with
an asterisk (*) show that they are default or
factory settings.
Setting the Volume Related to
the Guidance and Phone
The sound volume for the navigation can be set.
You can separately set the volume of the route
guidance and the beep sound.
1 Touch [settings] in the navigation menu,
then touch [Volume].
2 Touch [+] or [–] to set their volume.
On this screen, you can operate the following items.