Impress export to swf file unreadable

In Impress 64 bit for win 10 . HP Elite Book.
“Insert audio or video” , choose WMA file of voice just 532 KB as a test .
End Goal to narrate a stack of 30 slides all in one MP4 format file .
Test case use one slide .
Then “File Export” & save as, " macromedia flash ( SWF) "
resulting file small size . 39 to 66 KB . ( different attempts)
SWF Will not open or play in VLC . just a blank unchanging window.

Also checked on an iMAC & cannot open the swf file either .
" no suitable demux module" in monologue box
Crash log … input stream failed, cannot peek, nothing to play

Is there a recommended Free SWF reader & file converter to MP4 ?.
Adobe site says they do not support SWF so why is LIBRE using that ?
Thanks ! JerryM