There's always the faraday bag or simply not taking it to your meeting/activity that have been the recommended options for opsec even when phones had removable batteries.
If you turn your phone off and immediately put it in a Faraday bag, how would it receive a remote command to turn on and start recording audio?
Of course this applies whether it appears to have a removable "battery" or not.
That's always one of the big issues in opsec/security discussions, we can always imagine a more motivated or well funded attacker but the likelihood of those being deployed against you change with the difficulty of implementing those methods.
Edit: on the other hand, I'm now considering the possibility that the phone might be recording your audio even without network access and then transmit it later when you take it outside the microwave. So you'd have to be physically far away from the microwave for the microphone not to hear you, which means the microwave and faraday cages don't add anything useful.