This is the AltSFX sound set for OpenTTD (currently version 12). It tries to give its different feel while maintaining a CC-BY license.
At this moment it is complete. Feedback is appreciated (https://www.tt-forums.net/viewtopic.php?f=29&t=85650) . This sound set as a whole is under CC-BY 3.0 license.
Samples from the following sounds were used:
- "Fireworks Sound" by Stephan from soundbible.com - "public domain"
  used in 00.wav
- "Disappointed_Crowd" by unchaz from freesound.org - CC-BY 3.0
  used in 01.wav
- "Digging1" by cameronmusic from freesound.org - CC-BY 3.0
  used in 02.wav
- "Industrial/Factory Fans" by IanStarGem from freesound.org - CC0
  used in 03.wav
- "steam train horn 01" by eliasheuninck from freesound.org - CC-BY 3.0
  used in 04.wav
- "steam1" by gadzooks from freesound.org - CC-BY 3.0
  used in 05.wav
- "Boat Horn" by Alexander from orangefreesounds.com - CC-BY 4.0
  used in 06.wav and 07.wav
- "takeoff1" by granizo from freesound.org - CC-BY 3.0
  used in 08.wav
- "Takeoff 1 (Distant)" by cliftonmcarlson from freesound.org - CC0
  used in 09.wav
- "rocky_mtns_train_horn_moving" by polymorpheva from freesound.org - CC0
  used in 10.wav
- "AMBLM air conditioner rumble loop" by LudwigMueller from freesound.org -  CC0
  used in 11.wav
- "JacobsLadderLong2" by Halleck from freesound.org - CC-BY 3.0
  used in 12.wav
- "Air Steam Various short 01" by 2imitk from freesound.org - CC-BY 3.0
  used in 13.wav
- "Railroad Crossing Japan" by MrAuralization from freesound.org - CC-BY 3.0
  used in 14.wav
- "Car breaks down" by m1a2t3z4 from freesound.org - CC0
  used in 15.wav
- "Dumpster_Diving" by 1sticky8 from freesound.org - CC0
  used in 16.wav
- "crash" by sagetyrtle from freesound.org - CC0
  used in 17.wav and 19.wav
- "Explosion_17" by tcpp from freesound.org - CC-BY 3.0
  used in 18.wav
- "Till With Bell" by Benbocan from freesound.org - CC-BY 3.0
  used in 20.wav
- "door metal heavy push bar beep open close with security deadbolt unlock click2" by kyles from freesound.org - CC0
  used in 21.wav
- "jingle breaking news radio" by Thejack288 from freesound.org - CC0
  used in 22.wav
- "LAX Airport Landing Sound One" by FreeToUseSounds from freesound.org - CC-BY 3.0
  used in 23.wav
- "Helicopter Sound" by navaneetha kris from soundbible.com - CC-BY 3.0
  used in 24.wav
- "Truck engine" by Joseph SARDIN from bigsoundbank.com - CC0 / "public domain"
  used in 25.wav, 26.wav and 62.wav
- "school bus truck horn honk 50m away" by kyles from freesound.org - CC0
  used in 26.wav
- "old_dodge_truck" by roscoetoon from freesound.org - CC-BY 3.0
  used in 28.wav and 63.wav
- "Approx 850 - Enthusiast Audience" by lonemonk from freesound.org - CC-BY 3.0 
  used in 29.wav
- "Crowd Ooohs and Ahhhs in Excitement" by noah0189 from freesound.org - CC0
  used in 30.wav
- "in the smithy 2" by l0calh05t from freesound.org - CC-BY 3.0
  used in 32.wav and 39.wav
- "Jackhammer" by MrAuralization from freesound.org - CC-BY 3.0
  used in 33.wav
- "Air Horn Sound" by Mike Koenig from soundbible.com - CC-BY 3.0
  used in 35.wav
- "Bleat Sound" by Mike Koenig from soundbible.com - CC-BY 3.0
  used in 36.wav
- "Cow Mooing Sound" by Mike Koenig from soundbible.com - CC-BY 3.0
  used in 37.wav
- "Horse Whinny, Close, A" by InspectorJ from freesound.org - CC-BY 3.0
  used in 38.wav
- "Chainsaw cutting firewood" by VlatkoBlazek from freesound.org - CC-BY 3.0
  used in 40.wav
- "Sugar Pack" by sKydran from freesound.org - CC0
  used in 41.wav
- "Duck Toy Sounds" by sacha-rush from freesound.org - CC0
  used in 42.wav
- "Piston Loop" by sebastianlund from freesound.org - CC-BY 3.0
  used in 43.wav
- "Conveyor belt" by freemaster2 from freesound.org - CC-BY 3.0
  used in 44.wav
- "bubbles (1)" and "bubble (7)" by Glaneur de sons from freesound.org - CC-BY 3.0
  used in 46.wav
- "bubble (3)" by Glaneur de sons from freesound.org - CC-BY 3.0
  used in 47.wav
- "G39-15-Steam Catapult" by craigsmith from freesound.org - CC0
  used in 48.wav
- "Metal_03" by Q.K. from freesound.org - CC0
  used in 48.wav
- "bubbles (2)" and "bubble (2)" by Glaneur de sons from freesound.org - CC-BY 3.0
  used in 49.wav
- "Running Water" by harrybates01 from freesound.org - CC-BY 3.0
  used in 51.wav
- "wet catch" by ag_the_king from freesound.org - CC0
  used in 51.wav
- "wind_voice1" by thanvannispen from freesound.org - CC-BY 3.0
  used in 52.wav
- "Putting toy cars in a box" by SoundsForHim from freesound.org - CC0
  used in 53.wav
- "cutting_tree" by fkurz from freesound.org - CC0
  used in 54.wav and 56.wav
- "Jet Plane Flyby" by qubodup from freesound.org - CC0
  used in 59.wav
- "horn" by UncleSigmund from freesound.org - CC0
  used in 62.wav and 63.wav
- "20061105FurnaceFanAcceleration" by daveincamas from freesound.org - CC-BY 3.0
  used in 65.wav
- "Jungle, Tropical birds and insects" by mikevpme from freesound.org - CC0
  used in 66.wav and 72.wav
- "Lion Roar" by qubodup from freesound.org - CC-BY 3.0
  used in 67.wav
- "Monkey screaming" by Archeos from freesound.org - CC0
  used in 68.wav
- "train trip loud 1" by Yoyodaman234 from freesound.org - CC0
  used in 71.wav
- A sound made by the author (ALEXbr from tt-forums) - CC0
  used in empty.wav