M4B files in iTunes without the quality we expect. So we can draw a conclusion that iTunes can only convert very little part of Actually we know that most of M4B files in iTunes are DRM protected. MP3 is not in high quality as the original M4B file.
To convert M4B to MP3, many people may recommend to use iTunes to convert M4B to MP3 directly, but this method can only convert the M4B files that are not DRM protected.