Good luck!
Level 17's background audio was originally created by The Caretaker, Misplaced in time.
It can be found through his album Everywhere at the End of Time.
Bandcamp profile and album linked below:
The Caretaker Bandcamp: https://thecaretaker.bandcamp.com
Everywhere at the End of Time: https://thecaretaker.bandcamp.com/album/everywhere-at-the-end-of-time
Last but not least, all the painting images (except main) were originally painted by William Utermohlen.
Wikipedia: https://en.wikipedia.org/wiki/William_Utermohlen