It’s not currently possible to select alternative audio source for AV1. So you need a cable that connects the headphone jack out of your consoles to the audio pins on the scart head. You can probably mod your existing cable yourself quite easily (just make sure you disconnect the mono audio source!).
If you would like the functionality you suggest in FW (if I understood correctly, make AV2 audio input source selectable together with AV1 video source), you can make a post in the features request section. I’m not sure there’s some technical limitation to implement this feature, since it could be expected this would have been implemented already.