With thanks to Phistachio on XDA Forum, check out the best guide I've ever seen for releasing a soft-bricked i9100...
http://forum.xda-developers.com/showthread.php?t=1457458
Oh Thank the stars there's someone talking about this issue with JB ROMs!!! I'm running SlimBean 4.0.4 on my i9100 (Galaxy S2) simply because every time I throw anything Jellybean on it, I lose the ability to hear phone call audio over my Eclipse branded head unit in my 1994 model Mazda. Like...