Nobody actually heard the explosion within 60 miles of the explosion. That is because the explosion was so forceful, it ejected all sound from the volcano into the air and the sound arched back down to the ground within about 60 miles from the center. Not too many people lived to not hear the volcano, but the few who were within range only heard the snapping of trees.
They have information about the Zone of Silence at the Mt St. Helens Forest Learning Center. I recorded a little of it in the beginning 00:19 seconds in:
I was always in awe over the sound barrier. But The projection of sound in such a forceful way that people can’t hear it is very interesting as well.
Here an author goes into more detail of the situation: