You will need an audio editor. This page gives instructions for using audacity's 'noise removal' tool, which does wonders with mucky audio. Remember to keep a copy of the raw, unedited audio just in case.
Contents
[hide]Step 1 - The Sound of Silence
This step 'trains' the computer by explaining what it is that you'd like to eliminate. First, elect a stretch of silence (the longer the better) to the computer can build a profile of what to remove. Then select the Effect/Noise Removal... option and click on the first button of the dialog button which it brings up:
Step 2 - Removal
Once you've done this, that profile lasts as long as the program is running. The final step is to select the region you wish to clean, and choose Effect/Noise Removal... option and click again. This time you should click on the second button in the dialog. The three sliders are worth fiddling with to change how the noise is removed. I've found they work quite well in the positions shown:
How to improve it further
Check that the noise is the same throughout; sometimes the recording may have a different level of background noise. Sometimes, they'll adjust the microphone half way through if they notice the noise, in which case apply the above in two stages.
If the volume level is low, and I've needed to amplify the recording, I've had better results adjusting the volume first and then removing the noise second, rather than the other way round.
This is not all you can do with Audacity. There's a bunch more which I haven't got my head round. If you're making your own recordings, check that the microphone is suitable (good frequency range, say 100-15000Hz), and that it's pointing the right way (if it's directional).