Sound like a problem with the Zumo motorcycle mount. There are a number of threads on the subject of mounts going bad. I haven't had a power problem but two mounts have had one or both channels of audio go out. I am now on my third mount. Garmin only replaced the first one.
You can try cleaning the contacts on the Zumo and the mount. A pencil eraser or electrical contact cleaner are a couple options. Otherwise you may need to look for a new mount.