Settings window -> Events tab -> New Event button -> Create a new event from scratch
Set the Description. Set Trigger to A level starts. Leave Conditions empty. Press the New Action button and add the sound you wish to play.
This will play that sound at the start of any round, including breaks. If you want it to be only rounds, set the Conditions to isRound. If you want it to be only breaks, set the Conditions to !isRound.