How do I turn up the volume in street navigation?

Depending on the unit there are a number of ways;
700HD/s: there is a volume knob on the top right hand corner of the unit, flick it up to turn it up, it will return to centre after each flick.
For other models this is done in software, either on the main menu screen (volume, top left) or in street navigation.
If in street navigation the unit boots up to a menu screen (with Find, Route, Manage and Setting buttons being shown) press settings>sound and adjust the master volume slider to suit.
If the street navigation boots up to a map, in the top right corner there is a small blue (almost) rectangle, if you tap this it will show the quick menu and along the top there is a slider bar that can be adjusted to your liking.