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[Gweilo]

Some audio sources, especially cassette rips, have hiss, hum, static.

You can often improve such audio using the Audacity sound editor.
It's freeware, get it from http://audacityteam.org/.
And also get the Lame encoder to save back to MP3: http://manual.audacityteam.org/o/man/faq_installation_and_plug_ins.html#lame

See https://support.audacityteam.org/repairing-audio/noise-reduction-removal gives a full explanation


There are two "effects" that are helpful:

First is Noise Reduction


There's a video example of how to do it :


(Skip the part about making a recording, you already have the audio files so just open it.)
If it's a stereo track, zoom in and look at the waveform. If they're identical, then on the "track" click the triangle button, choose "split stereo to mono" then delete one of  the tracks.

At the bottom left you will see the "Project Rate (Hz)". This is the sampling rate. Default may be 44100 (same as CDs).  I set it at 22050, makes the files a bit smaller, this is speech, not hi-fi.

You can adjust the strength of noise reduction, I select a few seconds, apply it, listen and then undo (control-Z).

If you do it too much it begins to sound "hollow".

When you have a good compromise, reduced noise but voice still sounds natural, select the whole track and apply.

If the audio is muffled, you can then try the "Equalization" effect and boost the treble.
There is a bunch of preset profiles, try "telephone" for a start.

The second effect is Noise Gate
See https://manual.audacityteam.org/man/noise_gate.html

Use this when you have a hum, hiss or static you hear between words.
You set the "Threshold", the volume of "silence" and this then makes anything with that level or less truly silent. There is an analysis function to help you find the level.

This may be enough by itself to make the audio much better.
From their manual: "Best practice: Use the noise gate after applying noise reduction. This way, you can use less aggressive noise reduction settings, which may grant you a cleaner end result."

Note that the Noise Gate is limited as to the size of the audio it can process. Maybe 13 hours.
So for long books, split it in parts and process each part separately.

Export to M4B

When you're happy with it, export your cleaned audio from Audacity as a high quality MP3 -- Preset "Standard, 170-210kbps" is good.
Then chapterise and then tag the MP3 files using MP3tag.

Finally, use fre:ac to make an M4B from the folder of MP3 files.

You can also save the Audacity "Project" files if you want to work on it again later. These will be quite large, equivalent to wave files, several GB for a book. Don't save and reopen MP3, you will lose quality every time.
But after you've made the MP3, you can cut and join it losslessly using MP3DirectCut if necessary.