Does Construct in someway support or do you plan to support .m4a files? From experience in encoding mp3 to m4a you get fraction of filesize and barely noticeable sound difference. If you use original RAW audio than you get max quality vs size. I can re-encode a non music file 1/4 size before I notice any difference and 50-75% size with music file, but its never staticy or grainy like a wma just some instruments sounds are in different pitch/tone. From a raw file there is no reason not to use m4a aside from its new and mostly supposed on cell phones.
Same for mp4 video files also.