Add the feature for the character to make "noises" when taking damage, and from what damage you get dictates how loud your character makes. It doesn't make sense for my character to be a silent badass and take a broken leg like a champ. At the least when taking any significant damage you should be screaming in pain. This adds one more level of consideration when fighting zombies, if you fight zombies you run the risk of getting hurt and potentially attracting more zeds to you. This doesn't apply to exercise fatigue.
This is how I'm planning this would work out.
Minor pain/ scratches - Character gasps or grunts similar to a whisper range.
Pain/ Laceration - Sound level should be higher than whisper but lower than yelling
Severe Pain/ Deep wound - Yell in pain
Agony/ fracture & Burns - Scream in pain and continuous grunts until medical treatment
Traits to accommodate the feature: Suggested by Furizu
Positive: Stoic [-4] Able to endure pain more silently
Negative: Wimp [+4] Doesn't like and can't stand pain.