If a musician model is slain, another rank and file model belonging to the same unit, but that is not a command group model, will retrieve the fallen instrument and take their place. Therefore, a unit cannot lose its musician unless it, the standard bearer and the champion are the only remaining models in the unit. This applies even if the musician is targeted by an attack that allows a specific model within a unit to be targeted, or finds themselves lying underneath a template.
If the only other remaining models in a unit are the champion and standard bearer, the musician must be removed as a casualty before either.