ffmpeg can easily translate all kinds of multimedia files from one type to another. After installing it, simply open up a console, navigate to the directory with the files you want to process, and type:
ffmpeg -i inputfile.aif -f mp3 -ab 192 -ar 44100 outputfile.mp3
[-ab specifies the .mp3 bitrate you want. '-ar 44100' is the standard sample rate used in digital audio, just in case the input file is non-standard.]
To tag multiple files at once, you can use a "for" loop:
for file in *.aif ; do ffmpeg -i $file -f mp3 -ab 192 -ar 44100 $file.mp3 ; done
This will process every .aif file in the directory. Unfortunately, the input filenames will carry over as-is, giving you 'inputfile.aif.mp3' as your output file name. But you can easily fix the extension afterwards with the rename command:
rename s/.aif.mp3/.mp3/ *.aif.mp3
If anyone knows of a better way to take care of this, I'd like to know.
You'll have to use an external program such as audiotagtools to add the id3 tags to your finished files. One of the easiest ways to do that is to make sure your .mp3 file name includes all the info you want in the tags, something like "Pink_Floyd--Dark_Side_Of_The_Moon--05--Money.mp3", then use the "tag from file name" function to read that into the tags.