Surely there was angry muttering.

Multiple people raising objections at once, each at a reasonable volume to speak to a large seminar room, and other people speaking to their immediate neighbor. If there was angry muttering, I don't recall it. But yes, a crowd of mathematicians going wild is still pretty chill.

There was a story that Fred Richman got involved in a wrestling match as a result of a disagreement over a proof one time.

I never heard the full story ... everybody in the department at NMSU where he was at the time would just make comments to each other of the form "Remember the time ...?" with a response "Fred's a beast".

Perhaps a chair tipped over at most.