From the rulebook:

Players will be required to maintain volume levels above minimum settings, which will be clearly marked on the controls. LCS officials may require players to adjust their volume levels higher if the officials determine, at their sole discretion, that volume levels are too low. Headphones must be placed directly on a player’s ears, and must remain there for the duration of the game. Players are not permitted to obstruct the placement of headphones by any method or place any item, including hats, scarves or other articles of clothing, between the headphones and the player’s ears.

What's up with this? Why would Riot want to require LCS players to play with audio on?