Wow...this thread is like, oldsies!
This is just one of many Beach Boys tidbits we'll never know the truth about.
When Brian was on 20/20 Downtown in 2000 (I think, might have been 2002), he was asked if a beating from his father caused his hearing loss. Brian said, "No, I was born that way."
A few years later when he was on Larry King Live, Brian said the hearing loss was caused by Murry hitting him with a 2 x 4.
He was lucky. Could have been a 4x4. Those mothers are lethal.