So, I've been playing for a while now and started to realise some things:
As you get used to the distant roars of the bacteria they no longer send chills down you spine, and act more like a localisation feature. What still gets your heart going is the terrible scream once it sets it's eyes on you.
I suggest that once you enter level 0, the bacteria will be completely silent until the first time it has line of sight with a player, or is close enough to hear the footsteps.