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.)