Here is my 100% Achievement Guide with timestamps!
Hope this helps :)
Intro - 0:00
Act 1 Start - 1:30
Collectible 1/10 (Drawing) - 1:40
1. A New Dimension - 2:30
Collectible 2/10 (Poster) - 4:00
2. Illuminated - 4:15
Collectible 3/10 (Postcard?) - 4:35
3. Not That Kind Of Game! - 4:45
4. To The Heroes Among Us! - 5:25
Collectible 4/10 (Letter) - 7:50
Collectible 5/10 (Letter) - 8:35
5. Room 529 - 9:00
Collectible 6/10 (Poster) - 10:10
Collectible 7/10 (Letter) - 11:00
6 & 7. Now That's Thinking With Shadows & My God, Its Full Of Stars! - 12:40
Collectible 8/10 (Huge Note) - 13:50
Collectible 9/10 (Letter) - 15:45
8. Punching Bag - 16:20
9. You Broke My Finger! - 17:05
Collectible 10/10 (Letter) - 18:20
10. Streets Sweeper - 18:30
11. A Carousel Of Broken Dreams - 22:00
Act 2 & Collectible 1/6 (Photo Frame) - 22:20
12. The Family Life - 22:35
Collectibles 2 & 3/6 (Letters) - 23:40
13. This Circus Is A Disaster & Collectible 4/6 (Postcard) - 24:25
14. The Cyclops Bottle - 29:10
15. Just Like Harry - 31:30
Collectible 5/6 (Note) - 39:00
16. No Backpack Bird Needed - 43:40
Collectible 6/6 - 49:00
17. GroundsKeeper - 49:25
18. Handy Person - 50:00
Act 3 - 50:40
Collectible 1/10 - 54:40
Collectible 2/10 - 56:35
Collectible 3/10 - 57:00
19. How'd You Get In? - 57:45
Collectible 4/10 - 1:00:25
Collectible 5 & 6/10 - 1:01:05
Collectibles 7, 8 & 9/10 - 1:01:35
Collectible 10/10 - 1:02:25
20. Bookkeeper - 1:02:40
21. Let There Be Light - 1:07:20
22. FIN - 1:08:30