This can be worked around by simply typing in the full path of a FLAC file, but it will undoubtedly inconvenience people who don't think to try that. Could it be added to the list of files shown in the "import sound" dialogue box in the next update? Thanks in advance!
ETA: this seems to be OS-specific, e,g., it imports them on Windows but not on Mac, so it should obviously only apply on OSes where it imports FLAC. (“Make it import FLAC on other OSes” is probably a bit overly ambitious for now.)