>>29457949
I'm not sure, but I'd venture to guess it has to do with air being used as fuel for the fire, the heat created a convection cell in and around the fire, and a large amount of the air around him was being driven up, so if his phone was above him he'd have sounded less muffled.
Probly.
Also on a similar note, I'd bet the reason you can see him start to stumble and shake before he collapsed was from oxygen loss.