A speaker is a special type of circular region, which is like putting a speaker on the ground, and then deciding how far away people can hear it. The nearer you get to the speaker, the louder it will sound. The outer border of the speaker is where the sound will start to play very quietly, the centre of the circle is where the sound will play the loudest.
When you walk into the range of the speaker, that is an event that can trigger sounds or images in the same way as other regions.