We have made a mic input adapter/connector for the GoPro camera’s USB port. With a small modification, this can be used for the Garmin Virb USB port for power input, audio out and audio/mic in.
You can buy the connector here.
The connector looks like this:
In the bottom picture you see a resistor mounted. By mounting a 2,2k resistor there, the connector will be identified to the Garmin and the pads will be as shown in the picture above.
You don’t need to remove the existing resistor, but can just mount the 2,2k resistor over the existing one. The mounted resistor is 330k, so if you mount a 2,2k resistor over it, it will result in a 2,185k which is less than 1% from the target value (and very few resistors have that precision anyhow).
If you prefer to use a standard hole mounted resistor instead of a tiny surface mounted resistor, you can easily mount a larger resistor between the GND pad and pin 4 at the connector.
I have not tested this myself, but according to the documentation, this should work.