The paper is about sending voice commands using lasers, not sound. There's also no mention of "root access", the paper is just about replaying audio-converted-to-light rather than having any kind of interactive "root" shell prompt. The only feedback to know whether the exploit succeeded would be sending commands (e.g. unlock a door), which the victim might notice.
"Command Selection: ...Purchase a Laser Pointer"
Making other people buy stuff with laser beams... wait until marketing finds out!