I was going to make this point in this post, but I decided to start a new one because I think this point stands on its own.
There are plenty of actual definitions of "sounder," but none that actually make sense for the name of a professional sports franchise. Over at the Goal Seattle Museum, a story from 1974 asks the question "what is a Sounder?" without actually answering it.
So, lacking a better definition, and using as the image of the franchise a symbol that is more geographic than anything else, a Sounder is someone from Puget Sound.
What is a Sounder? I'm a Sounder. We are Sounders.