every species has problems....humans rape and have long periods of helplessness when young....why would musth be any more crippling to a society?
That's a good point.
I think that the issue is that if whole cities get destroyed once a year when the male elephants go on a rampage, it's going to be difficult to stick around.
However, I don't think the existence of this would prevent sentience from happening in the first place, long before permanent structures are built. And by the time they are, it's likely the Mammuthans figure out how to live with it, or possibly even control it:
* naturally occurring sedatives (are there any poppy species native to North America?)
* really deep pits with no way out, except when the afflicted individual can prove that they are feeling okay again.
* some sort of meditation exercise which allows the elephants to learn to control their instincts when they feel the symptoms coming on.
Alternatively, if they find a way to control it with medication, perhaps some civilizations might even figure out a way to induce it when they want to during times of war.
It's also possible that, in a sentient species, musth looks more like manic periods in humans. By that I mean, it isn't just knocking things down: there's also hallucinations, paranoia, obsession, even depression... Probably more dangerous, except for among the few individuals who can learn to focus their energies to become extremely creative during those phases (once a year, one particular Mammuthan turns into the equivalent of Van Gogh).