I had the same problem today with a 12 Pro Max.
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It's a bug in iOS 18, and apparently it wasn't fixed in iOS 18.2, because that's what I'm running.
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discussions.apple.com
Damn irritating, as this the first time I've ever had this problem in all my iPhones since the iPhone 3G.
Some people got the audio back by sending it back to themselves in iMessage or else importing it into iMovie, but neither of those options worked for me. Some people had a problem where their audio was set for Spatial Audio but I don't even have that option so that's not my problem either. The video just has no audio. I also checked the sync'd video on my Mac, and that doesn't have the audio either. It's really frickin' aggravating.
Your beta testing sometimes just sucks, Apple.
EDIT:
I've since sent a bug report to Apple. I suggest you do the same.
BTW, I rebooted the iPhone and then tried recording new videos. The new videos have audio, but the old one without audio still has no audio.