The diseased mouth is found in the sewer level.
The maze will be different for each attempt
. I recommend searching for the mouth during your "One of Three" play through. The mouth can only be found in the narrow
hallways, not the larger tunnels. Look for the pink, throat-like hallways. Like I said, each level attempt is different, so the location will be different for everyone depending on how many times you've played the level. Just follow the pinkish hallways to the end and you'll find it in a matter of minutes. Good luck!
Here's a video of where I found the mouth on my first
attempt. Look for tar on the floor in the hallways to be lead to the mouth.