Audacity (free, GPL Win, Linux, Mac) to Normalize the audio if the m4b audiobook volume is too low. This is very rare that you'll encounter such a case.

First export from freac. In General Settings under Output filenames put

Filename pattern: <title>

Each chapter will be encoded and output to matching the chapter name.
Uncheck Encode to a single file
Encoder choose opus 32kbps or as high as the original bitrate as these files need to be re-encode three times in total.

1) in Audacity import all chapter files at once. Select All (Ctrl-A) then Effect | Normalize.

2) For that rare chapter if the wavelength doesn't go up like the rest double click on the track to highlight both tracks then Effect | Amplify and choose an Amplification (db) of whatever kind of matches the height of the other track waveforms.

3) File | Export | Export Multiple and choose mp3 or ogg. Don't lower the bitrate lower than the original.

Using Audacity seems a tad quicker to use for normalization than Ocenaudio next post.