This video has some really neat audio in slow motion.
1) How did they make this effect?2) Is it possible to make with HitFilm?
Effect starts at 0:12.
AFAIK, HitFilm does not have ability to slow down audio speed. For that kind of stuff I personally would use Audacity.Import audio into Audacity, then choose EFFECTS/CHANGE SPEED, in "Percentage Change" type something like "-30", then SAVE, then IMPORT into HitFilm
You can try using the Rate/Speed tool in Hitfilm on your audio or/and video.