New sounds for SCP mob

Because spooks deserves top quality spooking

Intended to be 3 kill sounds and 4 sight sounds
Added new sounds to SCP mob

- SCP will use his scare chord sound when he has a target and picks a
way to go to him (It might play multiple times, although it should only
play when he picks the target turf)
- SCP plays a neck snapping sound when he downs his target

The sounds may stack a bit, I'm not sure why
Fix sound picking

- Will now pick in the full array of playable sounds
Merge pull request #1077 from dylanstrategie/SCP-173

New sounds for SCP mob