Are you using headphones or an amplifier?
Approximately how old is the module?
Ok, the "Backup Battery Low" message is an indication that the internal battery is running down.
The battery is a CR2032 type battery and can be found at most all electronic supply stores or online.
The internal battery is mounted on the main circuit board and the TD-8 would need to be disassembled to access it.
To fix the sound problem, try plugging and unplugging headphones several times in quick succession and let me know if there is any change?
If you are still getting no sound, then the module has a fault, possibly with the sound amplifier circuitry, and it will need to be returned to Roland for repair.