I found this developer's technique https://stackoverflow.com/questions/15557933/receive-audio-via-bluetooth-in-android to accomplish this, although he says it can't be done with an app.  I am not a developer, but if this works maybe someone can find a workaround that could allow an app to accomplish the required changes at least on a rooted phone.

I think it is pretty clear why this feature is not available, it is just another way of stopping us copying copyrighted material, although there are still plenty of ways to do it which are actually more convenient than streaming would be.