Charles Ellicott Commentary Exodus 28:35

"And it shall be upon Aaron to minister: and the sound thereof shall be heard when he goeth in unto the holy place before Jehovah, and when he cometh out, that he die not." — Exodus 28:35 (ASV)

And his sound shall be heard. —Rather, that its sound may be heard. The great object of the bells was to make known to the people, by a sensible manifestation, every movement of their representative, every act that he performed on their behalf. The bells enabled them to follow in their thoughts the entire service that he was engaged in, to join their prayers and praises with his, and offer to God a common worship. So important was this union of priest and people in the worship of God regarded, that death was pronounced upon the high priest who should minister in the sanctuary without this essential garment.