What is inside an MP4 from a phone
An MP4 recorded by an Android phone, a Galaxy, a GoPro or a screen recorder holds an H.264 or HEVC picture track and, almost always, an AAC sound track at 48 kHz stereo and 96–256 kbps. Only that sound track is read here; the picture is skipped, so a 4K video converts as quickly as a 720p one of the same length.
Some MP4s carry Dolby Digital (AC-3) sound, typically TV recordings. Browsers cannot decode it, so those files give the original sound as .mka and no MP3; the file list says so before you press Extract.
Choosing the MP3 bitrate from the source
- Source AAC up to 128 kbps (calls, lectures, most social clips): MP3 128 kbps, or 96 kbps mono for speech only.
- Source AAC 160–192 kbps (most phone videos): MP3 192 kbps.
- Source AAC 256 kbps or more (music, concerts): MP3 256 or 320 kbps.
The row shows the source bitrate before you convert. Going far above it only makes a bigger file: the MP3 is decoded from the AAC, so detail the AAC already removed cannot come back.
How much smaller the MP3 is
An MP3's size depends only on bitrate and length: 192 kbps is 1.44 MB per minute and 128 kbps 0.96 MB, whatever the video's resolution. The picture is nearly all of an MP4's size; in a test, a 2.08 GB video with 7.5 minutes of sound gave a 9.09 MB audio file.
That is also why size limits elsewhere (250 MB to 1 GB on upload sites) don't apply here: the video is read from your device in pieces and only the sound is kept in memory.
Converting on Android and Galaxy phones
Open this page in Chrome or Samsung Internet, tap the box and choose the video from the gallery or from Files. The MP3 is saved to the Downloads folder; music players and the My Files app list it there.
Long videos are decoded in chunks on current Chrome for Android, so an hour-long recording works without running out of memory. If a phone browser is older, choose a section or use MP4 to M4A, which needs no decoding at all.
When M4A is the better download
Every MP3 made from an MP4 is the result of two lossy compressions, AAC first and MP3 second. If your player, phone, car or editor accepts M4A (iPhone, Android, Windows 10 and 11, macOS and most cars from the last decade do), the untouched .m4a offered next to each MP3 keeps the exact original sound at a smaller or equal size.
Keep MP3 for older MP3-only devices, some podcast hosts and tools that ask for MP3 by name.