@yi thanks for taking the time to try fixing it, and sorry for me suggesting the normalise filter.

No way should one have to spend the time to do manual spike removal. It's a mindless repetitive task and that's what computers are for. I googled a bit more and found a plugin that actually does the job for the whole file at once: Pop Mute (search it here).

I downloaded it, installed it, enabled it, and applied it your audio, and it worked great.

So here's what I did:

1. applied Amplitude gain of 10dB to whole file (need allow clipping on) (the audio was a little low so this helps)
2. applied Pop Mute to whole file, with the following settings:

Threshold: -8 dB
Mute Level: -80 dB
Look ahead: 5 ms
Release time: 200 ms

The white noise from some of the spikes is still audible but so low in amplitude that it's OK.

Let me know if you want a copy of the result, but you can get the same by following the above instructions.

Also thanks for the slides.