The bug (and the fix) were originally reported here: kivy/audiostream#37
I ran into the same issue and it's not obvious how to properly fix it. One reply to the bug report above says to manually copy the audiostream .java files over, but those get wiped with the next clean.
How do I fix buildozer or the audiostream recipe so that it copies the audiostream .java files into dists/myappname/src/main/java/org as it should?