There Has To Be Something Awful In Them

[Jackalope]

Didn't Buffet already state explicitely that audits are not a hindrance for releasing the information?

I'm fairly sure everyone has stated that.  I suspect the reason Trump won't release them is that he isn't as rich as he says he is.  Since he reacts like a petulant child whenever someone questions his wealth, I'm pretty confident in my assessment.

I think it's likely that, or that his returns would show he's not very charitable, or that he pays very little in taxes.

...though I think your supposition is the more likely.