Mishnah Tamid 3:8 lists a bunch of things in the Temple that could be heard all the way in Yericho.
Some of the things: the gate opening, the Levi'im singing, the Kohein Gadol on Yom Kippur, smelling the k'toret.
Now, could these really reach all the way from Yerushalayim to Yericho? Were they just really loud, was it a miracle, or perhaps the Mishnah is exaggerating.
The distance is apparently around 15 miles (25 km).
Your question is discussed in אהלי שם - המקדש - משניות תמיד, מידות
He points out that according to Harav Noeh the distance is 38.4 km. The Meleches Shlomo says that hearing the sound of the great gate opening and the ability to smell the ketores were miraculous. The Tiferes Yisroel can see no point in a miracle and therefore concludes that it must have been natural. The Meleches Shlomo points out that the purpose of the miracle was to emphasize the holiness of Jericho.
In פירושי הראשונים עמ''ס תמיד ח''ב the Ra'avad points out that there were no mountains or other objects to interrupt the sound. The Ra'avad quotes מורי הרב החסיד to say that it was a miracle because of the importance of Jericho.
In the same sefer, the peirush אוד לנתיבתי can be found. He brings the same views but offers the suggestion (on p 307) that it was not the voice of the Kohen Godol that was heard but rather the response of the people ״ברוך שם כבוד מלכותו לעולם ועד״
(It might be worth considering that the world was a much quieter place in earlier times.)
Many years ago, I was part of a team of engineers that were helping the US Army to determine the propagation of sound in the atmosphere. We found that pockets of humidity and/or cool air caused a lens-like effect that caused the sound to be focused at specific locations almost as if the source of the sound was adjacent to the receivers. This focus effect was noticed to work for more than a mile near small bodies of water. That was as far as we were measuring, but it seemed like it could work for larger distances. I don't know the geography of the Jerusalem/Jericho region, but a cool pocket of air near the ground between them might have had the effect of focusing the sound. But 20 miles seems a little far.
At all events there is a physical phenomenon on which the quoted effect might just be based.
