Remember, once you attempt to repair the cabinet, all bets are off and you may void the warranty. I would mail a certified and regular postage letter to the manufacturer and audio shop. Document phone calls and attempts as well. If you don’t receive a response, I would leave comments on every social media platform possible, share your experience, and warn people about the seller and manufacturer. I would bet a paycheck that if you leave negative comments on Instagram, Facebook, etc., someone will reach out within hours.
If all else fails, find a custom woodworker, not just a regular cabinet shop, to repair the crack with structural epoxy - possibly T88. It’s hard to determine the best method without examining the speaker cabinet. Best of luck!