Hello and Welcome to Just Answer. My name is***** can help you with your question.
- Have you tried to Resetting the GPS back to Factory Settings?
Most of the time that would bring back the Voice Features....you can try resetting the GPS while plugged in to the car, that way if you do get the Voice back and working then at least it would be worth thinking about spending money into a new battery.
- About the battery, Amazon.com has it for a $33.00 dollars, but you might need someone to install for you unless you think you can do that yourself....here is the link for the battery.
Please let me know if you need any further help.