I’m sorry I can’t be of more help here. I’m a hobby musician and mostly record electronic instuments, so there is no noise problems there. At work we do tv audio post production and have found the iZotope RX tools to be very useful in “sound restoration”. We use an older version of the package, I think the separate “restoration” tools are now packaged in a product called Advanced Audio editor. We found the tool “Dialog Denoiser” to work quite well, but I’m not sure what’s the name of the product where it is included now. Also at the moment we are experimenting with Zynaptic Unveil for removing unwanted room echo from interviews and it seems quite a promising product.
But enough with that commercial stuff, you wanted open source tools I would suggest as Edward Diehl did earlier in this thread that you try the Noise Removal tool that is built in Audacity. Here is a video of how it is used:
Basically using the Noise Removal tool is a 2 step process.
First you need to tell the tool what you consider to be unwanted noise in the recording. This is done by selecting a short area of the recording where there is only unwatented noise and nothing else. Then you open the Noise Removal Tool and click on the button “Get Noise Profile”. The tool now inspects the selected area and what frequencies appear there.
Then you select the whole recording and open Noise Removal tool again. This time you select how many decibels you want the noise to be attenuated. Be careful with this setting since removing too much noise quickly brings in some “hollow” sounding artefacts in the audio. Start with a low value like 6 dB’s, and if that is not enough undo and try a higher setting. Noise removal makes you choose between the original noise and the sometimes quite nasty sounding artefacts that the noise removal process creates. You need to balance between those
A shotgun mic might not help in a noisy environment, especially if you are recording indoors, since the noise tends to bounce off the walls and around the room. When you turn the mic away from the noise source, you might get it again reflecting of a wall
At the moment I use Gentoo Linux, before that I used UbuntuStudio happily for a couple of years. I don’t think that you need any fancy plugins for the stuff that you do. In tv audio post production the main tools we use are eq, compressor and limiter. These will get you most of the way there. It might help to put each speaker on his own audio track, so that you can eq and compress each person separately. Then you probably need to record some volume automation for each audio track to level out volume jumps and make each speaker as loud as the others. Sometimes you need some noise removal (that might be done with audacity) and sometimes some de-essings for taming those sharp s - sounds.
I’m just beginning to explore the plugins that are available for Ardour, but at the moment I use EQ10Q as my eq (or it’s smaller version EQ4Q), SC4 compressor from Steve Harris, Invada Early Reflection Reverb (for adding room echo in music), ZamExcite to create some more high frequencies when needed (for musical instruments).