Short, sweet, and juicy. The sound design was spot on in creating the atmosphere and a sense of urgency, though it could be enhanced to the next level with proximity-based zombie sounds.
Something you may consider for next-steps is lighting, or rather, a lack thereof. Give the player a flashlight to see in their forward direction, but otherwise have the level only be lit by the alarm's emergency light.
Also, the volume was a little startling on startup, so perhaps a volume slider with defaults on the lower end. Otherwise, great work, and good luck!