Formats
MP3 or MP4: which one should you pick?
One keeps the picture, the other throws it away. The interesting part is what that does to size, quality and where the file will play.
The short version
- MP4 is a video file: picture and sound together. Pick it when you need to see what is happening.
- MP3 is audio only. Pick it for music, podcasts, interviews, lectures, or anything you will listen to rather than watch.
- An MP3 made from a video is typically ten to thirty times smaller than the MP4 it came from.
Converting to MP3 does not lose the sound quality
A common worry is that extracting the audio degrades it. In practice the soundtrack in the video was already compressed at upload, and pulling it out keeps that same quality rather than adding a second round of loss.
What you cannot do is go the other way. Turning a 128 kbps track into a 320 kbps file only makes it heavier: the detail was discarded when the original was encoded, and no converter brings it back.
Where each one plays
MP4 is the safest video container there is. Phones, TVs, editing software and messaging apps all accept it without conversion, which is why it is the default here.
MP3 is just as universal on the audio side: car stereos, old players and every music app read it. Formats like OGG and Opus compress better at the same bitrate, but MP3 is the one that plays everywhere.
A practical rule
Ask what you will do with the file in a week. If the answer involves watching, editing or sharing a scene, keep the video. If it involves listening while doing something else, the audio file is smaller, faster to download and easier to store.