Check out old Pride events (or any older Japanese MMA.) Everybody is quiet (with a few isolated shouts of encouragement) until someone does something heroic, and then there's a polite and energetic round of applause. The only reason that Zuffa UFC sounds like it does is because they intentionally tried to steal audiences from US professional wrestling. They also spent years standing people up almost immediately when they were jiu-jitsuing each other because the wrestling audience would just start booing aggressively after about a minute, the result being that the UFC were very kickboxer and greco-roman focused and some real killers had all their weapons taken away from them by UFC's application of their "Unified Rules."

Japanese MMA was founded and branded by people who were saying that Japanese professional wrestling was too theatrical, and Zuffa UFC was branded by people who were saying that professional wrestling wasn't violent enough (if anything, they were competing with "backyard" wrestling.) UFC has improved since, but imo that's because it became a monopoly and had to absorb all the other MMA audiences (and fighters), and the wrestling fans who didn't get bored with MMA eventually got less stupid.

> to me, making noise during a tense, violent event seems far stranger.

I also don't think there's any safe assumption of how Colosseum crowds behaved other than how contemporary narratives say they did. I agree that life and death brings an atmosphere of seriousness that wouldn't often exist at the Circus.