If still no sound on crossing the limit, please check if iPhone or 3G iPad are not connected to bluetooth or FM transmitter and their output is not muted. For bluetooth in your car, please also make sure it is in the mode when music from iPhone can be transmitted, sound alert from the speedometer goes through the same channel.
2. If app is not on screen when you drive, please be sure to allow the app to work in background* when screen is locked for example, or some other app runs as the top most one. To do that tap on MENU and then on "WORK IN BACKGROUND". When you don't need the alert, just tap on MENU and "DON'T WORK IN BACKGROUND", so GPS receiver would not be draining battery with no reason.
*Ability to work in background is only available in the full version.
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