Sound System

Goal
Offset the silence of your ride, for safety reasons, and also for style.
SoundRacer
This plugs into the 12V accessory port (OEM or otherwise) and provides a small fake ICE vehicle sound for your Zero.
This is meant for cars so it uses a radio frequency for a car radio to tune to.
A mobile app is provided for customization and control.
No reported Zero installs, for the obvious overhead of installing a stereo system on one's motorcycle.
SoundRacer EVEESS
This sound system is definitely DIY: be prepared for some light crimping work and to pin connectors and install a speed sensor on the front axle.
For the Zero, mind the size of the speakers; the smaller speaker pair are easier to mount than the single large speaker, unless you have the OEM crash bars.
Preparing a custom sound file involves using a simple utility to process it for the embedded computer. Prior to that, you'll want to use an audio-processing tool like Audacity to fit your sample into its requirements.
NanoMech's installation report