The use of WMA must be a legacy hangover from the early days of BIAB on windows, before widespread adoption of MP3.

It must be be quite painful, I imagine, to maintain one audio format for windows and another for mac, when AAC or MP3 is cross-platform and with much lower filesizes for same quality.

Well, at the end of the day, WMA works, but would be nice to have the faster downloads and smaller install that a newer codec could provide.