Problem Adding Sound to Slide Transition, Impress 6.1.0.3

Windows 10 Pro 1803 OS build 17134.228
Dell Optiplex 790
Two Impress Files
A - 80 slides 95 MB
B - 24 slides 84 MB

Both incorporate Sound from Other Sources during slide transitions.
The Sound files are located in the same directory as their Impress files.

A - was originally built with Apache Open Office in October, 2017 - BEFORE v. 1803 was installed.
It uses two .wav files, each edited with Audacity.
The first is a 1Khz test tone. It is typically up for only 10 seconds or so.
The second is a 100 second file from the opening to 2001: A Space Odyssey.

B - was originally built with LibreOffice 5 in June, 2018 - AFTER v. 1803 was installed.
It uses two .wav music files, each edited with Audacity.
The first file runs for 4-5 minutes.
The second for about 90 seconds.

File A worked fine, throughout 2017, but now has NO sound.
File B works fine, every time.

File A appears corrupted somehow. NO sound.

I’ve removed both Sound files and saved the presentation with no audio, then closed LibreOffice.
Then, reopened the file, added the sounds, saved. No joy.
REPEATED with every combination and permutation I can think of.
I’ve tested everything else I can think of.
I’ve even used Audacity to convert the files to .mp3.
Added them, instead. No joy.

NEXT?