M4B, a form of AAC audio file, is a widely used format for audiobooks because of its bookmarking capabilities, and is the format for audiobooks from the Apple iTunes store. M4B may not be supported on older devices, though. To use your M4B audiobooks with older music players, you can convert them to the widely supported MP3 format. In Ubuntu Linux, you can do this using several free, open-source programs available from the native Ubuntu software repositories.
Click "Applications," then "Accessories" and finally "Terminal" to start the Terminal application.
Type the command "sudo apt-get install faad." Press "Enter." Enter your password if necessary, and press "Y" to confirm the installation if the dialogue asks you.
Type "sudo apt-get install lame" into the Terminal window then press "Enter." Press "Y" to confirm the installation if required.
Type "faad --stdio input.m4b | lame --preset standard - output.mp3," and replace "input.m4b" with the file you want to convert and "output.mp3" with the name you want the converted file to have.
Type all commands without the surrounding quotation marks. You may want to look at other presets for lame that provide higher quality for music or lower quality to save more space. Find out more by typing "lame --preset help" in your terminal.