If you have an iPhone or iPad you can use it as the ‘source’ for recording. That means you get sound and video. Very easy.
Just attach your iPhone/iPad to your Mac with the USB Lighting cable etc… On the Mac, choose ‘New Movie Recording’ from QuickTime’s File menu. Then choose your iPhone/iPad as video source from the drop-down arrow next to the red recording button in the QuickTime player window. Also choose for sound.
Sound is the only confusing bit … I can imagine why it could be this way … but anyway … You will NOT hear the sound during recording, however it is perfect via playback.
Once you have recorded what you want, save the file. If you want to convert to mp4, this is built-in to the MacOS. In the Finder, click on your mov using the Contextual Menu and choose ‘Encode Selected Video Files’ from the Services menu. You will have a choice for quality settings etc. The resultant file for most choices, is an m4v file, which is Apple’s version of mp4, but includes DRM protection.