Unable to add music to play for duration of slideshow presentation.

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