As I think there is no ready-made cross-platform solution by Qt for doing this, the only way is to directly use native Android code (Java) to detect the Headphone state.
For example with a solution as described here: https://stackoverflow.com/questions/6249023/detecting-whether-a-headset-is-plugged-into-an-android-device-or-not (you can also look for other other solutions, frameworks, … )
To integrate native Java code with your Felgo application (QML) it is required to go through Qt C++, which allows to use JNI (Java Native Interface) to run native Java code. For more information, please see Integrating C++ and QML and the Android Extras / JNI features.
We can help with implementing this feature with the purchase of a Support Package to cover the required development hours. Just contact us via email at email@example.com if you’re interested in an offer.
Günther from Felgo