It might be humans making those sounds.
Keeping in mind that I am older than dirt here are two examples:
1-1970 International Hotel Las Vegas Elvis Presley
I watched the show from back stage where I saw three
guys each with their own SM 58 mic held up to their mouths looking into some sort of
lil Sony black and white (monochrome?) monitor waiting (if needed)
for a visual cue from Elvis to start singing.
Why three guys? I was told because up to that point
Elvis had three major music periods in his life and each guy could nail one style.
At that time in 1970 it was the Viva Las Vegas Elvis.
The stress all came to a head when the end of the show came and the high note in "My Way’
might become an issue. But that night Elvis was in fine vox and he nailed it.
2-1977 Toronto David Bowie and the Ziggy/Spiders/Mars show.
I knew the house crew so I got to be backstage.
Four great looking/sounding young musicians out front with david,
four additional musicians (old guys) back stage.
Live is sometimes all about the illusion but you can’t pull this kind sh!t in a bar.
You’ll get a beer thrown at ya!