Share sound causing microphone to turn on (with Zoom rooms)

Hi all, 

I have 1 person in a team of 40, who when he shares screen to a zoom room (Macbook pro - i5) unable to share sound

the room account is doing the Mic/video, he clicks 'share sound' nothing comes through to the meeting room. 

if he tries to turn his sound up; the microphone/speakers instantly turn on and start causing a feedback loop

Anyone come across this issue before, none of the other laptops do that in our setup, and we can't figure out whats going on.