given that most peoples audio equipment (headphones, speakers etc) would not be able to reliably produce sounds outside of the human hearing range across all the various games host systems then I find it very unlikely that they would use an inaudible audo file because it would defeat the purpose of it not actually being detectable and listenable...
Record in game sound with a capture device or program, feed the audio through some sort of program/device to see if there any frequencies there?
You can play music through an oscilloscope https://www.youtube.com/watch?v=iERih_7ZvP0
What's stopping you from playing a recording to see if there is any reaction on screen?
Who even records by putting their microphone next to a speaker nowadays anyway?
There are plenty of ways to record audio you can't hear and be able to visualise it.
I'm not an electrical engineer but I know things like this can be done.