We have a successful graphics replacement project going on at the moment, but nobody seems to have thought of that other important thing - sound effects. While currently there is no "new sound" format (with the exception of sticking them in GRF files, I suppose), I did write a very simple "sndcodec"-type program a few years ago that would decompile and recompile the sample.cat file. I'll try to tidy this up and release it soon.
I would suggest, if possible, we try to put together a series of replacement sounds for the default TTD sounds. Some of these can likely be engineered ourselves (eg, some of the UI sounds - the morse code newspaper notification, the "splat" as you place tracks, etc). Others we may need to record ourselves or source from free sound libraries (http://freesound.iua.upf.edu/ is a good place to look - I've spotted a few sounds which may work well for TTD, with some editing, such as a level crossing replacement). I recall reading that the graphics are likely to be licenced under some sort of Creative Commons licence. Seeing as many sound libraries are under a CC licence, it would make sense for our new library to be under the same thing.
If people are interested, then please do volunteer yourselves here! A few words about quality and so on: ideally, all our sound effects should be at least 44,100Hz, 16-bit mono. (Due to TTD's panning of sound effects, any stereo effects would be lost). Currently, the sound engine doesn't support anything more than 11KHz 8-bit, but that can (and will) change.

I hope to post up my "sndcodec" program soon - I have exams next week so it may be a little while. Until then, here's a list of the sound effects we need replacements for. It may be worth transferring this to the wiki somewhere.
Note that the descriptions are those from the sample.cat file itself, mistakes and all. Also, not all of the effects appear to be actually used in the game.
0. Good year
1. Bad year
2. Splat
3. Factory Whistle
4. Train
5. Traing through tunnel
6. Ship horn
7. Ferry horn
8. Plane take off
9. Jet
10. train horn (I think)
11. Mining machinary
12. Electric spark
13. Steam
14. Level crossing
15. Vehicle breakdown
16. Train breakdown
17. Crash
18. Explosion
19. Big crash
20. Cashtill
21. Beep
22. Morse
23. Skid (plane)
24. Helecopter
25. Bus start, pull away
26. Bus start, pull away with horn
27. Truck start
28. Truck start 2
29. Applause
30. Ooooh
31. Splat
32. Splat 2
33. Corrupt sound
34. Car horn
35. Car horn 2
36. Sheep
37. Cow
38. Horse
39. Blacksmith anvil
40. Sawmill
41. Rip
42. Extract & pop
43. Comedy hit
44. Machinary
45. Rip 2
46. Extract and pop
47. Pop
48. Cartoon sound
49. Extract
50. Pop 2
51. Plastic mine
52. Wind
53. Comedy breakdown
54. Cartoon crash
55. Balloon squeak
56. Chainsaw
57. Heavy wind
58. Comedy breakdown 2
59. Jet overhead
60. Comedy car
61. Another jet overhead
62. Comedy car 2
63. Comedy car 3
64. Comedy car start and pull away
65. Maglev
66. Loon bird
67. Lion
68. Monkeys
69. Plane crashing
70. Plane engine sputtering
71. Maglev 2?
72. Distant bird
I've attached the sounds in a decompiled form to this post. (Note that a few of the sounds are in 22KHz form, but are actually played in-game as 11KHz - if you decrease the speed by 100%, you'll get it as it's intended to be). If anybody would like to "claim" sounds to work on, feel free. Alternatively, if anyone has any comments or queries, do post them here!
Edit: Note also that I'm not suggesting we use the sample.cat format in a future version of OpenTTD - the whole sound/music engine really needs a redesign (something which I've conceptually begun doing, anyway). But for now, we can downsample our sounds to 8-bit, 11KHz and stick them in sample.cat for testing purposes.