Alarmed by this thread I just did my own experiments using “Audio Recorder” from FDroid.
The phone provides 3 audio sources, “Mic” “Unprocessed” and “Bluetooth”
Mic provides crystal clear audio if and only if the only sound audible is a single human voice (speaker in a silent room)
The mic audio source becomes unusable if there are ambient noises such as music. After a few seconds of recording the music, the audio quality heavily degrades, surpressing and amplifying random frequency ranges in a way that sounds “under water” almost as if several flanger effects were overlayed over each other.
It seems that the “Mic” source has a voice filter auto-enabled that behaves very badly if the audio source is not a single voice but music or anything else.
The “Unprocessed” audio source does not exhibit this behavior but has an extremely weak signal. To hear anything, I had to software amplify the signal by at least factor 4. At this soft gain factor there is a lot of of white noise audible.
Attempting to record from “Bluetooth” Audio source just reports “Source Disconnected”
What’s missing is a source with a hardware gain that doesn’t try to be too smart for its own good. “Unprocessed” has terrible signal to noise ratio due to lack of gain and “Mic” is unusable for anything but an isolated speaker.