no sound with videos on iphone 12 mini

Brand-new phone. FaceTime and other phone conversations work fine, but there is no sound with any videos, either mine or others. When I go to play back my videos, the mute button is always on, and unmuting doesn't work, so it definitely recorded with no sound. Video sound works fine on various iPads, and the mute button is NOT on after I record on them (successfully).

Same thing happened on my old 5SE, but I didn't need to make videos there so didn't follow up. In these COVID days now I DO need to!

(note, this is not an iPhone 11 but Apple hasn't updated the list of choices)