In order to compress aiff files on a PC you need shntool with the sox helper program. You can find it using the SHNTOOL link in my signature.

With shntool you can then convert the aiff files to wav format and use FLAC Frontend, or if FLAC's default compression level suits you, you can convert straight from aiff to flac using a command line like this:

shntool conv -o flac *.aiff