I've been looking into it, and one theory that doesn't sound completely insane is atmospheric ducting. Here's an official report from the Tallahassee National Weather Service, when this happened in Florida.
WWUS82 KTAE 100018 SPSTAE
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE TALLAHASSEE FL 718 PM EST WED MAR 9 2011
FLZ011-013>018-026-027-114-115-118-127-GAZ155>158-100230- JACKSON-CALHOUN-INLAND GULF-INLAND FRANKLIN-GADSDEN-LEON- INLAND JEFFERSON-LIBERTY-INLAND WAKULLA-COASTAL GULF-COASTAL FRANKLIN-COASTAL JEFFERSON-COASTAL WAKULLA-SEMINOLE-DECATUR-GRADY-THOMAS 718 PM EST WED MAR 9 2011 /618 PM CST WED MAR 9 2011/
…STRANGE SOUNDS BEING REPORTED ACROSS THE FLORIDA BIG BEND…
THERE HAVE BEEN SEVERAL REPORTS OF VERY STRANGE SOUNDS FROM THE WOODVILLE AREA SINCE ABOUT 645 PM EST. THE NATIONAL WEATHER SERVICE BELIEVES THIS PHENOMENON IS BEING CAUSED BY THUNDER FROM DISTANT LIGHTNING STROKES… BOUNCING OFF A VERY STABLE LAYER ABOVE THE GROUND. THIS IS CALLED DUCTING…AND CAN ALLOW SOUND TO TRAVEL UNUSUALLY LONG DISTANCES. THIS SOUND HAS BEEN MISTAKENLY ATTRIBUTED TO POSSIBLE TORNADOES…BUT WE AT THE NATIONAL WEATHER SERVICE WANT TO BE VERY CLEAR THAT THERE ARE NO TORNADOES … AND NONE ARE EXPECTED.“
And here's the main video of that event: https://www.youtube.com/watch?v=GEWTrYB-ERA
The only problem I have with this theory is that all the videos of this sort of sound event on Youtube have dramatically different weather conditions, and thunder doesn't usually last 20 minutes.