Unable to access microphone when another app is using it in Android

Sadly, only one app at a time can access the microphone (just like the camera).

Also, there is no standard way to inform another app that you want access to the microphone (so that they release the resources and let you access it). You could however send a broadcast to all other apps ("requesting the microphone"), but the other apps would have to implement this feature (and very few or zero developers will do this).

I would recommend you to simply inform the user that the microphone is currently not available, because you can't do anything else.

If you are using the "OK Google" function with the option to access it from any page, try turning that off (the 'any page' bit From Google app screen MENU>SETTINGS>VOICE>"OK GOOGLE DETECTION">FROM ANY SCREEN=OFF): it hijacks the microphone, or can do.