Using the Arma 2 keyboard preset (and not the default Arma 3 one), while being prone on the ground, I double tapped the "w" key and started sprint crawling.

The sound the game made was the wrong one, because the sound that I heard is something I would associate with crawling at slowest speed. Tthere should be three sounds for crawling (slow, normal and sprint).

Also I think the crawling sound sucks iimo, but that is an other issue.