If you need to mute a video file quickly, iMovie works great for this on Mac: open your clip, go to Modify > Detach Audio, then delete the audio track and export. On iPhone, the Photos app lets you tap the speaker icon to mute a clip before saving.
If you need to process multiple files at once, or you're on a Mac that doesn't have iMovie handy, there's also a free browser-based option at https://remove-audio.com ... it runs entirely in your browser using WebAssembly, so your video files never leave your device. Supports MP4, MOV, MKV, AVI, and WEBM, up to 20 files at a time.