I have two speech audios. One of them is very sharp and crisp audio . While the other has some humming with the speech. I am not talking about the static humm. The speech itself has a humming effect. I want to understand what is that makes these two speech sound different. And is there any way (using MATLAB, Audacity) to make the second audio sound crisp like the first one?
It sound like the good version has simply been cleaned up using equalization.
The humming you speak of is just bad low frequency 'rumble'. It's a bad recording, but in the cleaner version somebody has removed some of the more troublesome frequencies, and maybe boosted some of the brightness.
It would be pretty easy to tidy up the 'bad' version using the same, simple technique which would definately be available in Audicity and MATLAB.
That sounds like "musical noise", artifacts from extreme compression. If the people designing the audio compressor were not able to anticipate the characteristics of coherent speech better, chances are rather slim that your attempt on fixing the result after the pertinent information is not just gone but substituted by strange noise could be successful.