I don't recall testing layout-id 21 but I have tested these 2:
Use layout-id 16 if you want internal speaker sound, but rear audio jack doesn't detect
Use layout-id 15, both front and rear audio jacks work, but no internal speaker
If someone can combine them 2, you got a winner!
For now, layout-id 15 is good enough, internal speaker isn't the best quality anyway.
Update:
Tested layout-id 21, both front and rear jack works but no internal audio, so same as layout-id 15