Welcome to WikiProject Media Restoration, a project dedicated to improving Wikipedia's presentation and use of historic sounds, still images, and moving images.
WikProject Media Restoration seeks to provide high quality media related to composers and songwriters by locating and restoring freely licensed recordings, scores, and portraits. Could use more examples.
If you're new to restoration, we'll help you get up to speed.
Archival searches
These include adding unused, high-quality files to appropriate articles; locating high-quality, pre-restored files under a free-licence; documentation; and propagating high-quality content. For instance, if one of us finds a repository of high-quality music, such as [1], [2], or [3], there may be some research necessary to get sufficient documention on the recording, and to will also need to be distributed throughout appropriate Wikipedia articles.
I've had good results with this method, using the freeware program Audacity:
Find a patch of "silence" - that is, a few seconds that have no sound except for the hiss or crackle you want to remove. If necessary, create a patch of at least a second at the end by copying shorter gaps in the music. Be careful to only include silence in here - not the fading notes of the last chord, etc.
Hit cntrl-2, Cntrl-1, Cntrl-1. This will get you at a good zoom level for the next part.
Go through, listening to the song, and, except in your patch of "silence", remove as many pops as you can by highlighting and deleting them. Don't highlight too much, or you change the tempo. Check after you removed them, because sometimes this can make things worse. If there's a large amount of crackle, just get the really, really obvious ones, then, perhaps, run the click removal tool under "Effects" as an additional step (it... sometimes helps, particularly when there's too many pops to remove by hand without changing the tempo, though it's often useless) and go to step 4.
Under edit, choose Select->All, then go back to the edit menu and choose "duplicate". You now have two identical tracks.
Select the patch of silence on one of these tracks (you may want to zoom back out using Cntrl-2), then go to the effect menu and choose Noise removal. Hit the "Get Noise Profile" button.
Select all of one of your two identical tracks. Home, followed by Shift+End will do this for you. Go to Effect->Noise removal and, using the default settings, remove the noise.
You now have a cleaned track and an uncleaned track. This is because noise removal removes information, so you don't want to remove the noise completely. Instead, using the slider bars on the left of each track window, adjust the relative volumes. I find that no change or -3dB to the cleaned track, and -6db or -9dB (depending on how noisy the original was) to the uncleaned is generally a good choice. You need to combine the cleaned and uncleaned, because removing hiss also removes information, and without adding some of the information back in, voices and instruments can begin to sound a bit unnatural or just generally lower quality.