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#lameSee
http://wiki.audacityteam.org/wiki/Noise_Reduction gives a full explanation
There's even 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.
When you're happy with it, save to MP3: (This is what you need the Lame plugin for)
File/Export audio
Choose "type" : MP3 file
Option: choose bitrate. I use "Variable", "Quality 9" (the lowest, but I find perfect for speech) and "Speed: fast " (this is actually the newer, better method, as well as faster.) And "joint stereo", if it's mono, this has no effect.
You can use a higher quality if you prefer, but compare the results and see if you really get any noticeable improvement for the filesize increase.
Then to make a set suitable to torrent,
chapterise and then tag the MP3 files using
MP3tag.
However, M4B is the preferred audiobook format now. To make those, save your cleaned audio from Audacity in a high quality MP3 -- Preset "Standard, 170-210kbps" is good.
Chapterise and tag the MP3s as above then
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.
