I came across a video on youtube http://www.youtube.com/watch?v=8oSbiIsc_8o and the sound was so horrible that it fascinated me. I downloaded the video, extracted the audio with ffmpeg and then tried to improve the audio with Audacity.
I failed.
Is there someone who has an idea what can be done? What filters, effects to apply on this audio?
The main distortion is digital compression-artefacts , either the up-loader or YouTube has applied a low-quality low-bit-rate , ( low kbps ). IMO this is not correctable.
Maybe in an other audio-editor?
No. There is little that can be done to improve that audio.
The original audio before uploading to YouTube may have been better.
There is an impolite phrase that comes to mind that means: “The impossibility of trying to make something hopelessly weak and unattractive appear strong and appealing.”
The phrase that I was thinking of was not exactly “mud” 
though I see on Google that some have found ways to polish all sorts of materials. Hmm, probably best to leave the conversation at this point.
That video suffers one major common problems… room reverb. You’re not going to be able to remove that. A more focused mic next time will be able to fight that better.
There are also secondary issues like noise from a poor wireless connection as well as improper sensitivity… clothes rubbing and ‘plosive’ sounds are now permanent. The speaker in the latter half is also badly mumbling and losing capacity to direct air from his mouth well. Again, not correctable through an editor.
Personally, I think the quantization errors from compression actually enhance this presentation a little bit.
Okay, I got it. It was worth the try, though. It was funny how you guys came up with creative answers. 
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