By listening to the first few seconds of chapter 1, I see there are a few problems that need to be fixed first:
The MP3 format is in 192 kbps. In Audacity, change this by going to File>Export>Export as MP3>Format Options and set the Quality to 128 kbps.
Volume falls too low in the dB scale (86-92 dB and it is currently at 80 dB). You can fix this by going to Effects>Amplify and bring it up by around 8 dB.
There are a lot of clicking and popping noises in the recording. Highlight one or two of these, go to Effects>Noise Reduction>Get Noise Profile, then click Ctrl+A to select everything (on a PC) and click Effects>Noise Reduction and plug in Noise Reduction=10, Sensitivity=3 and Frequency Smoothing=1.
Once you fix these, I'll be happy to give it another go