I'm new to MusicBee but what I've found out so far is great!! Thank you, Steven, for that awesome piece of software.
Well, Silas, the only freeware/Free Software (Open Source) I know that are able to convert from AIFF to FLAC (because it is told in the descriptions of these programms) are
1.) winLAME and
But I haven't tried it. So it's up to you.