If it's sensitive enough to read a heart beat, then surely it can be used as a covert microphone?

The laptop also contains a normal microphone. You can't access this without root; if you have that you have permissions to access the real microphone.

I can access the IMU without root on my M4 mac running Sequoia

From testing, it seems to require me to press my wrists quite hard against the macbook to get a somewhat accurate reading on the heartbeat. Non the less a cool project and I wasn't even aware my macbook has an accelerometer.

The sample rate is only ~800hz (probably very difficult/impossible to increase)

The fundamental frequencies of voice are below half that, I think that means you can use it to determine what is said writing it's sensitivity range.

Depends on the bandwidth.

depends on the bandwidth