This solution worked for me in iPhone 5s running iOS 7.1.1
While using the Maps application for directions, tap the bottom of the page and look for the speaker icon on the lower left before it disappears.
Tap that speaker icon and select the desired volume. Deselect "no voice". The speaker icon is only only present when Maps is being used for directions rather than simply as a map. Problem solved.
Maps in iOS offers turn-by-turn navigation. To use turn-by-turn navigation, the start location must be Current Location. When en route to your destination, Maps will announce directions, provide en route updates, and estimate the time to arrival. If in an area or on a device for which spoken, turn-by-turn directions aren't available, Maps will track your location and provide onscreen directions without announcements or en route updates. At any time, tap Overview in the top-right corner of Maps to display your entire route.