Add cave water drip ambience sound effects from Frogatto

Sounds by Adam Rinsky (ActionJack), licensed under CC-0. The original
PCM files can be found in the Frogatto repository.

Used by the SOUND_LIST:CAVE_DRIP macro.
Add cave background noise ambience from Frogatto

Sound by Jetrel, licensed under CC-0. The original version is in the
Frogatto repository; this one is actually slightly edited to make the
loop actually seamless.
E1S7: Improved ending cutscene transition
E1S7: Deploy cave ambience sound sources