If you have enough RAM, start with a ring buffer.

On interesting event: compress and transfer the relevant chunk of audio from the ring buffer back to Earth.

Interesting event trigger ideas:

1) loud sound after quiet time

2) manual timestamp request from control

3) video clip recording

4) midnight, sunrise, noon, sunset. These are mostly so you have some daily baseline.

5) science package running

6) rover moving

7) abrupt camera change