MPEG-4 Audio Layer bookmarkable data
File type: Audio and sound file
The mp4b file extension identifies MPEG-4 Audio Layer bookmarkable data used with Apple QuickTime.
These files contain audio with bookmark information, making them suitable for audiobooks, podcasts, and other spoken-word recordings.
File type category:
Audio and sound file
Date updated: October 9, 2026
The mp4b file format is an MPEG-4 audio format related to files using the m4a. Its bookmarkable form can store audio together with information that allows playback to resume at saved positions.
Bookmarkable MPEG-4 audio is commonly used for audiobooks, podcasts, and other audio works in which listeners may need to return to particular points. The default file extension for MPEG-4 Audio Layer bookmarkable files is m4b.
Open an mp4b file with Apple QuickTime. The software linked to this format is available for Microsoft Windows and as Apple QuickTime for macOS and Mac OS X.
If QuickTime does not recognize the file, the uncommon mp4b extension may be the reason; do not rename it unless the application or the file provider confirms that it is MPEG-4 bookmarkable audio.
Conversion is not normally necessary if the file plays correctly in QuickTime. When a different audio format is required, use QuickTime or other compatible tools to export mp4b files to other formats, where supported.
Exporting may remove bookmark information, so retain the original mp4b file if saved playback positions are important.
Apps designed for playing mp4b files:
Microsoft Windows
Apple macOS / Mac OS X
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