I'm currently trying to enhance a bad audio source I have. It has a nasty humming sound in the background and some clicks/pops/whatever it's called. Sometimes, there is also some weird echo with voices. As per recommendations in here, I previously used the WinAmp DFX plugin but need something more sophisticated. Can anyone point me out to an easy guide on how to clean up and improve bad audio sources with Audacity?

I've been using Audacity for some time. You must be sure that you highlight the entire track or you will not be able to use any of the alternatives on the menu marked "Effect". Once you do that you will find options to enhance and change your track. Some of the alternatives in Audacity are done much better in a software called "Groove Mechanic" put out by Coyote. Its not free but worth the small charge. In my opinion it removes "clicks and pops" much better than Audacity, which will do that but I think it alters the original sound quality too much, adding at times an echo effect.

You can experiment and try all the effects on the "Effect Menu" to see how it influences your original track. Be sure to number and save each "effect" so that you can call them up later in Audacity and compare them. One of the nice feature of Audacity is that you can open all of the tracks and have them side by side for comparison. Don't be afraid to experiment and save. You can always go back to the original track.

The one downside to using Groove Mechanic if you decide to get it is that it will only deal with a "wav." file. You can use Audacity to convert any mp3 file into a wav. file and then bring it into Groove Mechanic.

why don't you just use the noise reduction filter that comes with audacity? If all else fails you could also try the FFT filter too.

I'll try both those tools (assuming that Groove Mechanic has a demo version), but keep in mind that I'm completely new at this kind of thing and need easy-to-follow guides.

HotshotGG: are those two filter under the "effect" menu?

Although I don't use Audacity for my work, perhaps I can make a few suggestions that can be applied.
I see that Audacity has only one control for NR. I presume that is the amount of reduction. The various programs I use have a fair number of parameters that effect NR operation. That means there is more to learn, and more to adjust, but results may be much better. If you limit yourself to Audacity, then the following may let you get the best results the program can provide.
** You need a stretch of recording that has no music, such as between the individual songs on an LP. The very beginning or end of a side is generally not very representative so don't use that if anything else is possible.
** Listen to your noise selection at extra high volume level to be sure it indeed does not contain any music or speech. It must contain only the noise you hope to remove.
** Use a long enough selection to make your noise profile. Several seconds or more is best if possible. If only shorter durations are available, you might create a new file/track and duplicate your noise selection a number of times into it so the NR sampling process can get a better "picture."
** Test the noise profile and the reduction setting before applying to your file. The best way is to select a low level segment of your audio, several seconds or so long, and apply the NR only to it. Then listen through the treated selection, starting before it and going beyond it, to be sure that you are not harming the audio you want. Use undo after every trial, continue testing until you have the best settings before applying to all of the recording you want treated. Doing this on a number of different selections, where different instruments or other variations are present, may give you a much better idea of final results than testing only at one place.

NR will reduce hum along with the general background but it is not a very effective approach. Hum is generally at a much higher level, within a very narrow frequency range, than other noise. Trying to get the hum low enough with NR will often require so much NR that the music will suffer significantly.

Various kinds of filters (FFT, parametric equalizers, IR) are configurable so it is possible to filter out only narrow ranges at the correct frequencies. Many kinds of filters have been created as plug-ins. Possibly there are one or more that work in Audacity.

I did not see any transforms for treating clicks and pops in Audacity. Perhaps I just don't know where to look. I use several different programs. Like NR, click and pop removal generally comes with quite a few parameters that let it do the best job for the current audio. Click and pop treating plug-ins have also been created. Perhaps there are some that work in Audacity.