One small tip: if anyone yells out "we can't hear you back here", make a Vanna White-esque arm sweep showing the available seating closer to you.
My mom yelled this out at my shower and I spoke loudly for her co-workers for the next gift, and didn't care much after that. "THIS IS FROM SOMEONE YOU DONT KNOW AND IT IS THIS."
As for reactions, natural is best. DH and I were looking through the pictures from the gift opening and there are a ton of good ones and a LOT more funny ones.