Process:
First slide, select slide transition ‘none’, speed ‘medium’, sound ‘apert’ and the play preview button generates sound.
Now change sound to ‘other sound…’ and select my FLAC-encoded audio file, which is 25.3MB, on the local HDD.
Selection is rejected, sound returns to ‘apert’ (or any other sound previously selected).
Same result with WAV file, and even a 4MB MP3.
The answer to the question here:
states that FLAC is one of the available formats.
LO v5.1.4.2 on Mac OS X 10.9.5