The dive team is breathing tri-mix gas containing helium. They would sound like chipmunks. This was alluded to when Abby was discussing the decedent's oxygen deprivation.
Whilst built-in breathing systems (BIBS) and rebreathers are both diving technologies, they are markedly different, both in function and purpose. Abby's calling the BIBS gas separation unit a rebreather is incorrect, as BIBS are externally served in a hyperbaric or saturation chamber, and monitored, and controlled by technicians, as the characters on the ship were clearly demonstrating.