Can Americans understand Old American? /s

Try for yourself! https://www.youtube.com/watch?v=CH-_GwoO4xI

If you want a trip, try listening to Scots: https://www.youtube.com/watch?v=le3cBRlWSE8

Ok after listening to the old English maybe I ought to try Middle English first

I'm not a Yank, but I know Old English is too different, so haven't bothered to listen/

Middle English is a lot easier, but still awkward in some places. We had to read The Canterbury Tales at school, and I remember getting the gist without having to make to much use of the references for obsolete words.

As to David Campbell's piece, I only missed a few words of the Doric section followed the tale of the pious / holly fellow who went out in the fishing boat, and only really missed the (possibly implied) stealing of the bucket (rather than the literal loss of the fellow overboard while baling).

I'm not a Scot, and in many respects Doric is a difficult dialect of Scots to follow. Now if that had been a conversation in a pub, or on the street (rather than the presented story), I imagine I'd have had more trouble.

Part of the problem with Old English is it isn't just the usual linguistic evolution over time like in most languages. English changed dramatically after 1066 and the Norman Conquest. In modern English there are more words of Romance origin than of the original Germanic origin. Hence people like Poul Anderson and his famous "Uncleftish Beholding".