What is the name of the application to enable "transparency mode" in bluetooth headphones?

Question

Bluetooth

Good afternoon. My headphones don't have a "transparency mode" where I can listen to the sounds of the environment, but sometimes it's very useful, for example, on the road. I want to implement it programmatically. There is an Android smartphone, there are headphones with a good microphone. You just need to somehow let the sound from the microphone into the headphones when you press the button on the headphone, as an option. To remap bluetooth buttons, as a rule, you need root and an application like Button Mapper for Android. I searched for such an application and did not find it. It is possible that the query was entered incorrectly in the search engine.

Desired Algorithm:
1. Press the button on the earpiece twice. Instead of a voice assistant, some application is launched that turns on the bluetooth microphone on the headphones and sends sound from it to the same headphones.
2. When you press the button twice again, this mode is turned off and everything returns as it was.

Answer 1

Normally, this is implemented in a microcircuit that is inside the headphones.
If you drive the sound from the microphone to the android and then back to the ears, there will be a big delay. Not to mention that the surround sound will be monophonic.
Theoretically, you need a mixer application that can add sound from two sources, that is, a virtual output is created in the system, the sound from your media player goes there, then the mixer adds the sound from the microphone there and transmits it to the real headphone output.
But offhand, I don’t recall that in android you can just create virtual inputs and control sound redirection.
For the sake of fun, you can do it in an analog way, before this was popular.
something like this: www.radiolamp.ru/shem/unch/89plus an amplifier.