How to make your voice like this? (Presumably helmet-radio)

[Trebor]

@2Spooks

… speaking inside a concealed helmet to his radio …

"Gas mask" effect?

Nearest I can get at short notice is an astronaut helmet effect … Remove all frequencies below 300Hz (using equalizer). Apply David Sky’s comb filter: frequency 444Hz, decay 0.04 Then leveller on “light”, ( leveller is in Audacity’s effect menu ).