rain_heavy.ogg (originally 2.ogg) — used for both rain and storm (stacked twice for storm)
rain_light.ogg (originally 3.ogg) — currently unused
  "Rain (loopable)" by Ylmir — https://opengameart.org/content/rain-loopable — CC0 1.0

thunder_1.m4a, thunder_2.m4a, thunder_3.m4a — cut from "Tonitrus.ogg" by Mysid
  https://commons.wikimedia.org/wiki/File:Tonitrus.ogg — released into the public domain by the copyright holder

thunder_4.m4a — "sfx100v2_thunder_01.ogg" from "100 CC0 SFX #2" by rubberduck
  https://opengameart.org/content/100-cc0-sfx-2 — CC0 1.0

None of these require attribution; credited here for provenance.

sfx/ — destruction one-shots (cut, mono, loudness-levelled)
  sfx/explosion_1–12.m4a — cut from "Explosions-LS100150/152/155.ogg" and "Explosion-LS100155.ogg" by Fg2
      (Toshogu Spring Festival, Nikko) and "Explosion_10.ogg" by tcpp, Wikimedia Commons — public domain
  sfx/debris, crumble, glass, wood, hit, metal_1–5 — "75 CC0 breaking / falling / hit sfx" by rubberduck
      https://opengameart.org/content/75-cc0-breaking-falling-hit-sfx — CC0 1.0
  sfx/metal_6–10 — slam_01–05 from "100 CC0 SFX" by rubberduck, https://opengameart.org/content/100-cc0-sfx — CC0 1.0
  sfx/metal_11–15, thud_1–8 — "Impact Sounds" by Kenney, https://kenney.nl/assets/impact-sounds — CC0 1.0

Round 2 (replacements picked from the soundboard ratings — launch, pop, rayzap and crash files were later replaced in round 3)
  sfx/launch_1–5 (thrusterFire), pop_1–5 (explosionCrunch), rayhum_1–5 (forceField), rayhum_6–10 (spaceEngineLow),
  rayzap_1–5 (laserLarge), rayzap_6–10 (laserRetro) — "Sci-fi Sounds" by Kenney, https://kenney.nl/assets/sci-fi-sounds — CC0 1.0
  sfx/gun_1–13 — cut from "Gunshot Sounds" by Tabasco (sks, cz, mosin), https://opengameart.org/content/gunshot-sounds — CC0 1.0
  sfx/splash_2–3 — splash_01/02 from "100 CC0 SFX" by rubberduck — CC0 1.0
  From SoundBible.com, public domain:
    sfx/traffic_1 + sfx/horn_1 — "Traffic Jam 1" (https://soundbible.com/1609-Traffic-Jam-1.html)
    sfx/traffic_2 — "Traffic Jam 3" (https://soundbible.com/1629-Traffic-Jam-3.html)
    sfx/alarm_1 — "Car Alarm" (https://soundbible.com/96-Car-Alarm.html)
    sfx/alarm_2 — "Vehicle Alarm" (https://soundbible.com/888-Vehicle-Alarm.html)
    sfx/crash_1 — "Crash Large" (https://soundbible.com/1172-Crash-Large.html)
    sfx/splash_1 — "Splash Rock In Lake" (https://soundbible.com/2100-Splash-Rock-In-Lake.html)
  From SoundBible.com, CC BY 3.0 (attribution required — keep these lines):
    sfx/airhorn_1–3 — "Fire Truck Horn", SoundBible.com, https://soundbible.com/2192-Fire-Truck-Horn.html — CC BY 3.0
    sfx/crash_2 — "Metal Debris Falling", SoundBible.com, https://soundbible.com/1099-Metal-Debris-Falling.html — CC BY 3.0
    sfx/rubble_1–3 — cut from "Bunker Buster Missile", SoundBible.com, https://soundbible.com/1847-Bunker-Buster-Missile.html — CC BY 3.0

Round 3 (weapons and crashes)
  Public domain / CC0:
    sfx/launch_1 — "Missile Launch", SoundBible.com (https://soundbible.com/1794-Missle-Launch.html) — public domain
    sfx/pop_1 — cut from "Flashbang", SoundBible.com (https://soundbible.com/1793-Flashbang.html) — public domain
    sfx/launch_2 — "Missile sound" (missile.wav) by mikeask, https://opengameart.org/content/missile-sound — CC0 1.0
    sfx/pop_3–5, rayzap_5–10 — "Sound effects Pack 2" by phoenix1291 / SwissArcadeGameEntertainment,
      https://opengameart.org/content/sound-effects-pack-2 — CC0 1.0
  CC BY 3.0, SoundBible.com (attribution required — keep these lines):
    sfx/mgloop_1, mgtail_1 — "Minigun" by Jim Rogers (https://soundbible.com/1920-Minigun.html)
    sfx/mgloop_2 — "Heavy Machine Gun Fire" (https://soundbible.com/1387-Heavy-Machine-Gun-Fire.html)
    sfx/launch_3 — cut from "RPG Plus Shrapnel" (https://soundbible.com/2075-RPG-Plus-Shrapnel.html)
    sfx/launch_4 — "Bottle Rocket" (https://soundbible.com/709-Bottle-Rocket.html)
    sfx/launch_5, pop_2 — cut from "Grenade Launcher 2" (https://soundbible.com/2140-Grenade-Launcher-2.html)
    sfx/rayzap_1 — "Ray Gun" (https://soundbible.com/1770-Ray-Gun.html)
    sfx/rayzap_2 — "Laser Cannon" (https://soundbible.com/1771-Laser-Cannon.html)
    sfx/rayzap_3 — "Alien Death Ray" (https://soundbible.com/1274-Alien-Death-Ray.html)
    sfx/rayzap_4 — cut from "Laser Machine Gun" (https://soundbible.com/1774-Laser-Machine-Gun.html)
    sfx/crash_1 — "Car Explosion" (https://soundbible.com/106-Car-Explosion.html)
    sfx/crash_2 — "Car Brake Crash" by Cam Martinez (https://soundbible.com/1757-Car-Brake-Crash.html)
  (Round-2 files launch_1–5, pop_1–5, rayzap_1–10, crash_1–2 were replaced by the above; gun_1–13 are no longer used.)