Currently on Android, It's not possible to record internal sounds without the microphone. This is because of no public API availability.
Even though in-game sounds can be recorded in root mode, this is out of the scope of this plugin as it may be removed from store without notice by google anytime if publishing using private API's. So our plugin won't use any private API's for reliability.
However, you can record the screen with the sound through microphone which is acceptable.
In upcoming updates we will come up with an option to add custom music incase if audio through microphone recording is not yielding better results.