Use Android jack as a transmitter rather than receiver

Use Android jack as a transmitter rather than receiver

While the phone is in call mode it seems to use the mic from the bluetooth
headset. What I'd like however is to convert the audio jack into something
of an input device completely.
Say a 3.5mm jack is plugged into the phone, on the click of a button in my
app it should be able to receive audio input.
I've tried:
audioManager.setBluetoothScoOn(true);
audioManager.setMode(AudioManager.MODE_IN_CALL);
With all the possible 4 combinations of these two commands. No effect
however. I don't even hear the output audio through the headset. How can I
programmatically read data from the headset microphone without having to
be on a call?