As you know, it's common for FE games to have the Sound Test feature. In fact, i think every game has had it since either Thracia or Binding Blade. Anyway, to unlock Sound Test in Echoes, what you have to do is, at the title screen, press up, down, left, right, up on the D-pad. You'll hear a little 8-bit jingle if you did it right.
Sound Test will then be found in Extras.