Good arguments, TeamBftw. The Air element has been established as having sway over sound and sound waves, though. The arguments you present are more of the "if, if, if" type than the generic type. Generically, people think of echoes as going through the air. Therefore, an Echo card belongs in Air.
I could see an argument for a Sonar card or Dolphin card being in Water. A Bat card definitely should go in Darkness, although an Echolocation ability arguably would revert to Air. I can't think of any useful game purpose for Echolocation, though, so that's kind of a moot point.