You should make sure that the GPS is not experiencing interference. This can be caused by tunnels and tall buildings. If this is not the issue then you may need to replace the antenna. Check the Troubleshooting Page for more information.
Try going into the settings and turning the volume up. If the settings do not help to increase the volume, then the speakers may need to be replaced. Follow the repair guide for how to replace the speakers.
Try plugging in the GPS to see if it will stay on. If that does not work, check if the cable is working. You can test this by trying out a different cable. If the cable is not the problem, then you'll most likely need to replace the battery. Check out the replacement guide for more information on how to replace the battery.