The MTF squad(s), all "listening" SCPs, and the player.
(From now on, I will refer every enemy that is affected by noise the player makes by "the listening")
Short version: The listening are attracted to sound.
Long version: The player can make noise in various ways (Firing a gun, running (in close proximity of a listening enemy), and making noise because of some SCP, e.g. coughing, screaming of pain (in result of some random SCP we don't have yet)). When making a lot of noise, (or when you notified a listening enemy by your noise), an eerie sound will play that indicates that you made a lot of noise and are attracting the listening. (a drum with a long echo with a screeching violin for example, like a cave sound from one very popular game.) focusing more on the MTF squads, this feature can also work the opposite way. You can hear the MTF squads communicate through radio (with those popular bleep sounds at the end), which will get louder when the MTF squad comes closer. You can hear an ambience (or heart beat sounds) that will also get more intense when the MTF are coming closer.
Any other notes: