I'd always assumed chris was talking horseshit when you said they recorded his screams, but now you mention it, it seems a bit too creative for his brain.
It could be that time and time again the teachers were explaining to bob and barb that their son sounded like Damian being dragged into church, his body going into full resistance against any logic, control, reason, or calming measures. So when enough was enough, they had to physically restrain him, and play this cassette of Chris belting out Cthulhu Worshiper's Greatest Hits Volume III.
Naturally they hear it and assume their kid is being tortured, because to them, being passive and letting Christian have what he wants means they'll seldom, if ever, hear their son unleash the cacophonies of a Slaaneshi noise marine.