Free online MOBI to AAC converter

Some audio players and apps accept AAC audio but cannot open MOBI ebooks. Converting an ebook for listening requires text-to-speech (TTS): MOBI contains book text, while AAC encodes sound.

What the MOBI format is

MOBI is an ebook format used for books read on older Kindle devices and in Mobipocket-compatible software. You may encounter MOBI files from ebook libraries or older digital-book collections. Mobipocket Creator, Amazon’s older Kindle publishing tools, and Calibre can create or export MOBI files.

What the AAC format is

AAC (Advanced Audio Coding) is a lossy audio codec used for spoken audio, music, and streaming. It is commonly stored in an .m4a file; some devices also accept raw AAC streams with an .aac extension. AAC can provide good audio quality at moderate bitrates, but it does not contain ebook text or formatting.

How to convert MOBI to AAC

This conversion synthesizes speech from the ebook; it does not recover recorded narration. The resulting voice and pronunciation depend on the selected TTS engine and language.

Online on 101convert.com

1. Open the MOBI-to-AAC converter on 101convert.com and upload the file. Several files can be added for batch conversion; check the page for its per-file size limit.
2. Before converting, set the voice language and reading speed.
3. Start the conversion, then download the generated AAC file.

Offline with desktop software

With free Calibre and eSpeak NG, extract the ebook text, synthesize it to WAV, then encode that audio as AAC with FFmpeg. These programs are available for Windows, macOS, and Linux.

1. Install Calibre, eSpeak NG, and FFmpeg. In a terminal or command prompt, convert the ebook to plain text: ebook-convert book.mobi book.txt.
2. Generate speech from the extracted text: espeak-ng -f book.txt -w speech.wav. The default voice may not match the book’s language; select an installed eSpeak NG voice with its voice option if needed.
3. Encode the WAV file as AAC in an M4A container: ffmpeg -i speech.wav -c:a aac -b:a 128k book.m4a. To create a raw AAC stream instead, use ffmpeg -i speech.wav -c:a aac -b:a 128k -f adts book.aac.

Quality and limitations

Plain-text extraction omits page layout and images, and may include material such as navigation or footnotes that needs cleanup before synthesis. TTS produces a computer-generated voice, not an audiobook recording. The AAC bitrate controls encoded audio size and quality; the example uses 128 kbit/s, which can be changed in the FFmpeg command.