Where MOV files come from
MOV is Apple's QuickTime container. iPhones and iPads save camera videos as MOV, macOS screen recordings (Shift-Command-5 or QuickTime Player) are MOV, and many mirrorless and cinema cameras write MOV as well. When an iPhone video is sent by AirDrop or copied to a computer it usually stays MOV; messaging apps often turn it into MP4.
The format is close to MP4, so the same reader handles both. Dropping an MP4 here works too.
HEVC video doesn't matter: the sound is AAC
With "High Efficiency" selected under Settings › Camera › Formats, iPhones record HEVC (H.265) video, which some Windows PCs can't play without an extension. The sound track next to it is ordinary AAC stereo, and this tool never decodes the picture, so HEVC, H.264 and ProRes MOVs all convert the same way and equally fast.
The MP3 is made from that AAC. The original AAC is also offered as an .m4a next to each MP3, which is the better pick for an iPhone, iTunes or Apple Music library.
iPhone spatial audio (APAC) tracks
iPhone 16 and later models can record spatial audio. The MOV then has an APAC track for spatial playback plus a regular stereo AAC track for every other player. No browser or open-source decoder reads APAC yet, so the stereo track is chosen automatically and the result notes that the spatial track was skipped.
The stereo track is what you hear when the video plays on a PC or is shared, so nothing audible is lost for ordinary listening.
Camera MOVs with PCM sound: keep it lossless
Many cameras and recorders (and iPhone ProRes clips) store sound as uncompressed PCM, often 48 kHz 24-bit, which takes 17.28 MB per minute in stereo. For those files each MP3 comes with a WAV of the original PCM, byte for byte (for sound tracks up to 256 MB, about 14 minutes), which is the right file for editing or archiving.
Choose MP3 only for listening or sharing; converting PCM straight to FLAC in the Video to MP3 hub keeps it lossless at about half the size.
QuickTime's "Audio Only" export compared
On a Mac, QuickTime Player's File › Export As › Audio Only saves an .m4a of the sound. That is fine for a single file on a Mac. This page also runs on Windows, Android and iPhone, converts many files in one go, gives MP3 for players that need it, keeps PCM as WAV and lets you cut a section first.