On many operating systems, I can create audio loopback devices to feed audio files into another app as if it was coming from a microphone. I can also tee off audio going out to the speakers.
Is there an app that lets me redirect audio devices to perform the equivalent of connecting audio inputs and outputs on Android?
Note to moderators: while such a feature can be used for piracy, that is not my goal, and there are better methods if I was trying to be nefarious. I want to use it to feed audio samples from soundboard apps into my active phone call without having to play the sound effects through the microphone. I'd also like to use it to directly supply the output of an MP3 into another app that records my voicemail message for better quality than can be performed by simply recording through the microphone.
Buy/DIY a USB-C to minijack adapter, and feed the headphone output (or any other analog audio signal) into the microphone input.
However, I'm unsure if you'll have to change the gain of the analog signal before feeding it back in.
A different solution could include Bluetooth mic input as the pure input source using a BT transmitter with a minijack input, but that will introduce some latency. However, the later versions of the protocol can work with a very low, possibly unnoticeable, latency.