Hi guys.
I have already recorded a lot of courses with my iphone (application italk) in aiff format with best quality.
Those files need a lot of space. I need to edit those files (remove noise, amplify etchh…)
Because i need space i want to convert to mp3 format.
I use audacity.
In witch quality i need to convert for not lose any possibilty to edit with good results.
i have no time to edit files now but i need space 
For speech quality 64 is considered a good bitrate, 128 will give you an excellent qualty for speech and a compression ratio of around 10:1 compared with your AIFF files. I sue 128 for my spoken word files
For music I would use a minimum of 192, but these days I personally use 256 which I find very good.
WC
MP3 compression damage never goes to zero, it just gets less as your bitrate goes up. Uncompressed stereo bitrate is roughly 1400, so that gives you the fuzzy conversion.
You need to know that MP3 makes a terrible archive format because it’s a dead end. You can’t pull your archive out and cut or edit a new version – or you can but you can’t ever go back to MP3. MP3 damage is cumulative and permanent. Each time you make a new MP3 the damage increases – sometimes doubling and you can’t filter it out.
You can always go from uncompressed WAV down to anything else – any other delivery format which is what MP3 is.
Koz
Copy the full quality recordings onto an external hard drive, CD or DVD as WAV or AIFF files.
For safety it would be best to make at least one additional back-up copy in full quality WAV format.