A positive note:
I was able to copy an audio file from the assets into the privateDir (with the File component) and play it from there with the Player component (of course without READ / WRITE permissions). Therefore it should also be possible to first download (audio) files to the ASD, then move them to the privateDir (copy / delete) and play them from there.
This could circumvent the problem of the AIA limitation to 30 MB and thus save sensitive files in the internal storage, which users would only have access to with a rooted device.
Note: I haven't found a way to decompile an AAB yet. This would be very helpful for adding (large, many) assets later ... as has been possible with the APK so far (see here).