I have been searching for couple of days now and havent been able to find a suitable solution.
I am trying to check if any app in the background is using the microph
You can do it the other way around.
Get the microphone in your app.
Get a list of the installed apps, who have a RECORD permission.
Then check if one of these apps is on the foreground and if there is one release the microphone so that the other app can use it (for example when a phone call occurs).
A bit dirty practice but I think it is what you are looking for.
Cheers!