I have checked the origin and original license of all the OpenSFX sounds and I created a nice list as an overview. Here's the list for your information:

• "1sticky8" from "freesound.org"
  • Dumpster_Diving.wav (CC0)
• "acclivity" from "freesound.org"
  • ChugChugWooHoo.mp3 (CC BY-NC 3.0)
  • TwoCows.wav (CC BY-NC 3.0)
• "AGFX" from "freesound.org"
  • Squeeky ball Toy_1.L.wav (CC0)
• "Aldor" from "pdsounds.org"
  • L'ascenseur* Elevator (Public Domain)
• "alexrigg" from "freesound.org"
  • crash_treefall_SE.mp3 (UNKNOWN LICENSE)
• "Anton" from "freesound.org"
  • wind1.wav (CC BY 3.0)
• "Benboncan" from "freesound.org"
  • Circular saw crosscutting.wav (CC BY 3.0)
• "benhillyard" from "freesound.org"
  • Vocal_Splat_08.wav (CC BY-NC 3.0)
• "Bidone" from "freesound.org"
  • Affen schreit.mp3 (CC0)
• "cats2009" from "freesound.org"
  • blue_angels.wav (CC Sampling Plus 1.0)
• "cfork" from "freesound.org"
  • boing_raw.aif (CC BY 3.0)
• "conny" from "freesound.org"
  • DATSUN_T.wav (CC Sampling Plus 1.0)
• David R Barnes ("earthcalling") from "pdsounds.org"
  • Corner of a sheep field in summer (Public Domain)
• "ddub" from "freesound.org"
  • concorde.mp3 (CC Sampling Plus 1.0)
• Derek Murphy ("robbiesurp" at "freesound.org")
  • wfl5.5_snap.wav (CC BY 3.0)
• "www.digifishmusic.com" ("digifishmusic" at "freesound.org")
  • Passenger jet departs 2.wav (CC BY 3.0)
• "dobroide" from "freesound.org"
  • 20060419.horse.neigh.wav (CC BY 3.0)
• Elaine Miller ("Miselaineous" at "freesound.org")
  • elaine-growl.wav (CC Sampling Plus 1.0)
• "Goldy-sama" from "freesound.org"
  • bulle.wav (CC BY 3.0)
• "Halleck" from "freesound.org"
  • JacobsLadderLong2.flac (CC BY 3.0)
• "han1" from "freesound.org"
  • Car start and drive.mp3 (CC BY 3.0)
  • claxon.wav (CC BY 3.0)
• "HerbertBoland" from "freesound.org"
  • MouthPop.wav (CC BY 3.0)
• huha from "tt-forums.net"
  • maglev_sound_2.wav (UNKNOWN LICENSE)
• "icmusic" from "freesound.org"
  • london bus approaches & leaves.wav (CC BY 3.0)
• Janis Lukss ("Pendrokar" at "wiki.openttd.org")
  • Own recordings/mixes (UNKNOWN LICENSE)
• "jascha" from "freesound.org"
  • kick_1.wav (CC BY 3.0)
• "JFBSAUVE" from "freesound.org"
  • CHAINSAW.wav (CC BY 3.0)
• Jillian Callahan ("JillianCallahan" at "freesound.org")
  • generic prop_start(8.395).wav (CC Sampling Plus 1.0)
• "joedeshon" from "freesound.org"
  • slide_whistle_down_fast_01.wav (CC BY 3.0)
• "krillion" from "freesound.org"
  • flyby.mp3 (CC Sampling Plus 1.0)
• "l0calh05t" from "freesound.org"
  • in the smithy 2.wav (CC BY 3.0)
• "Leady" from "freesound.org"
  • Dropping a large gun.wav (CC BY 3.0)
• Leon Milo ("milo" at "freesound.org")
  • msfinmarken_Bergen.aif (CC BY 3.0)
  • ship2_bergen.aif (CC BY 3.0)
• lonemonk from "freesound.org"
  • Approx 850 - Enthusiast Audience.wav (CC BY 3.0)
• "lorenzosu" from "freesound.org"
  • helicopterRaw_16sec.wav (CC BY-SA 3.0)
• "man" from "freesound.org"
  • swosh.aif (CC BY 3.0)
• "Marec" from "freesound.org"
  • metro.wav (CC BY 3.0)
• "Matias.Reccius" from "freesound.org"
  • crashB.wav (CC0)
• "Necrosensual" from "freesound.org"
  • aluminum02.wav (CC0)
• "NoiseCollector" from "freesound.org"
  • CRASH2.wav (CC BY 3.0)
• "patchen" from "freesound.org"
  • Locomotive 1 Distant horn.wav (CC BY 3.0)
• "Pooleside" from "freesound.org"
  • nnb04_maxed.wav (CC BY 3.0)
• Remko Bijker ("Rubidium" at "dev.openttdcoop.org")
  • Own work
• Richard Frohlich ("FreqMan" at "freesound.org")
  • whoosh06.wav (CC BY 3.0)
• Robert Gacek ("Robinhood76" at "freesound.org")
  • 01063 roadworks driller.wav (CC BY-NC 3.0)
• "roscoetoon" from "freesound.org"
  • rr_cross5.wav (CC BY 3.0)
  • t_start1.mp3 (CC BY 3.0)
• "Q.K." from "freesound.org"
  • Metal_03.wav (CC0)
• "sagetyrtle" from "freesound.org"
  • crash.wav (CC0)
• "saphix" from "freesound.org"
  • file0375.mp3 (CC0)
• "scuola_rocca_di_botte" from "freesound.org"
  • Manuel Tarquini.wav (CC Sampling Plus 1.0)
• "Sedi" from "freesound.org"
  • ae_51_m.wav (CC BY 3.0)
• "simon.rue" from "freesound.org"
  • Boink_v3.wav (CC0)
• "SlykMrByches" from "freesound.org"
  • splattt.mp3 (CC0)
• "Stickinthemud" from "freesound.org"
  • Bike Horn double toot.wav (CC BY 3.0)
• "suonho" from "freesound.org"
  • ELEMENTS_WATER_02_Phasin-bubbles.wav (CC BY 3.0)
• "tigersound" from "freesound.org"
  • bird tweet.aif (CC BY-NC 3.0)
  • bird tweet 4.aif (CC BY-NC 3.0)
• Timo A. Hummel ("Felicitus" at "dev.openttdcoop.org")
  • Own work (UNKNOWN LICENSE)
• Tom Haigh ("audible-edge" at "freesound.org")
  • Nissan Maxima burnout (04-25-2009).wav (CC0)
• "VEXST" from "freesound.org"
  • Snare 4.wav (CC BY 3.0)

Is this list good enough to update the README?