Free online M4B to SPX converter

Source format

M4B is an AAC-based audiobook container commonly used by Apple devices, iTunes, and audiobook apps. It is designed for long-form spoken audio and often supports chapter markers, bookmarks, and sometimes DRM protection. Typical uses include purchased audiobooks, podcasts, and other narrated content where preserving playback position matters.

Target format

SPX is a file format used for Speex, an older speech-oriented audio codec built for low-bitrate voice recording and transmission. It is optimized for spoken audio rather than music, and is often used in VoIP, voice archives, and applications where small file size is more important than high fidelity. Its main advantage is efficient compression for speech.

How to perform the conversion

The most reliable way to convert .m4b to .spx is with FFmpeg, because it can decode M4B audio and encode Speex directly. If your M4B file is not DRM-protected, the conversion is straightforward.

1. Install FFmpeg on your computer.
2. Open a terminal or command prompt in the folder containing the audiobook.
3. Run a command such as ffmpeg -i input.m4b -c:a libspeex -ar 16000 -ac 1 output.spx.
4. Check the result by playing output.spx in a compatible player or by inspecting it with ffprobe output.spx.

If you prefer a graphical workflow, you can first open the M4B in a media editor or converter that supports FFmpeg-based export, then choose File → Export or File → Save as and select Speex/SPX if available. However, direct command-line conversion is usually the most dependable option.

You can also convert the file on 101convert.com for free, without installing any software.

Notes on quality and limitations

SPX is a speech codec, so the converted audio will usually sound noticeably less detailed than the original M4B, especially for music, sound effects, or high-quality narration. For audiobooks, it can be acceptable if small size is the priority, but it is not a lossless conversion.

Some important limitations apply to this specific conversion:

• DRM-protected M4B files cannot be converted unless they are first legally unlocked or obtained without DRM.
• Chapter data, cover art, and metadata are usually not preserved in a simple audio codec conversion to SPX.
• Speex is an older format; compatibility may be limited in modern players compared with AAC, MP3, or Opus.
• For spoken audio, using -ar 16000 and -ac 1 often gives a good balance of size and intelligibility.

If you need the audiobook structure, chapters, or artwork preserved, consider keeping the file in M4B or converting to a more modern container/codec combination instead of SPX.