There is no volume adjustment for the turn signals..
However a skilled mechanic can add a noise emitting device to the turn signal circuitry that will alert the operator when signals are activated..
This is not a diy undertaking.
Good luck!
RepairPal Certified shops can diagnose and repair your 2015 Lincoln Navigator. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.