The additional description that would best help develop the character is: "Sonny’s focused practice swings made harsh whooshing sounds that caused nearby players to cast sideways glances at him, clearly impressed."
This detail helps to establish Sonny's skills and impact on those around him, painting a picture of him as a serious and talented player who stands out during tryouts. It adds depth to his character by showing how others perceive him and indicates a level of confidence and ability that could be significant to the narrative.