A lot of people think audio doesn’t matter when they edit video. They’re wrong. If you notice the audio, something’s wrong.
An easy way to improve audio quality is to normalize it, which makes the volume more consistent. Windows users can do it with Normalizer, a GPL command-line utility (a GUI is available). Linux users can use Normalize.
Normalizing still won’t make bad audio sound good, but it will at least make bad audio sound better.
Please may also want to look at Replay Gain for their music collections.
From what I can tell Normalize(r) recodes the audio which can add further noise to the signal. Replay Gain simply adds a tag to the files so that players adjust output levels. Thus you don’t lose any quality in recoding.
P.S. Would it be possible to add a “preview” button to comments?