On the off-chance anyone else is having the same problem of zero audio/mic input, I did some digging and trial/error and worked out the following:
- a reboot didn't help
- a reinstall of the app didn't help
- a reset didn't help
- wasn't just specific to this app, Shazam/VoiceNotes had no mic input either
The only way I could get it to work was to plug in the external headphones, and it then worked with the headset mic - from there, after unplugging the headset it worked with the built-in mic once or twice, but after that it was still dead with the built-in mic. Only thing I can think is that the jack is confused about whether the headset is plugged in, so something may be flaky with my phone.
If anyone else has had similar issues and sorted out a more specific fix, please let me know - otherwise this may help someone cut down their troubleshooting time...