Well, it’s not possible to create an app to send audio via bluetooth, at least not without a lot of delay. The bluetooth stack needs to be modified in order to allow the device (in this case the watch) to act as an HSP (hands-free profile) receiver.
The bluetooth stack is quite complex, and that’s why nobody has managed to enable this functionality yet. On Android 5.1 devices it was possible because the bluetooth stack was much older and known to developers.