Each mutalisk attack upgrade will increase its damage over base by 12.5%
Each mutalisk defense upgrade will increase its damage soak over base by 10%

A mutalisk, unless permitted no heal time, will be able to absorb one extra volley with +1 armour, and will be able to kill a goliath in 2 fewer hits with +1 attack.

Furthermore, against all other targets (tanks, SCVs) the mutalisk defense values are irrelevant