No Audio Was Recorded On My Phone.
Check if the thick phone case causes poor contact.
Make sure that the whole system has sufficient power and the mics are not muted.
No Sound to Hear when Using an iPhone/iPad with A Lightning Port.
Make sure to use the included 3.5mm TRS-TRRS cable (with a phone label) to connect to your Lightning-3.5mm adapter.
Check that you plug the TRRS end (three black poles) into your Lightning-3.5mm adapter, but not the TRS end.
It is recommended to use a Lightning-3.5mm adapter purchased from Apple.com because of the protocol of the Lightning jack.
If you happen to have a Type-C to Lightning cable that's also from Apple.com, it will also work. Just plug the Type-C end into the bottom Type-C port of the M9 receiver, then plug the Lightning end into your iPhone or iPad. Though it's not wireless anymore in this use case.
It Doesn't Pick Up Audio On My Android Phone
Make sure the system pairs successfully by checking if the blue light on the mic is solid on. If not, follow the "It doesn't seem to pair successfully" question above in the "First-time setup" section to pair it up.
Ensure that OTG on the phone is turned on.
Try downloading software like Open Camera to record audio while filming.