If you have sox installed, simply
while true; do sleep 1800; play -n synth 0.15 sine 800; done while true; do sleep 1800; play -n synth 0.15 sine 800; done brew install screenbrightness
while true; do sleep 1800; brightness 0; sleep 0.1; brightness 1; done while($true){(New-Object Media.SoundPlayer "C:\YourPreferredSound.wav").Play(); sleep 1800;}