In the early days of amplified sound, speakers were often conical, a shape that naturally directs and intensifies any sound going in the narrow end. Though speakers have become much more compact and often have their actual shape obscured by a case, the cone shape is still synonymous with loudness and amplification.
The marks coming out of the speaker represent sound waves, though only the presence of the loudspeaker symbol makes these differentiable from, say, wireless signal marks.