The variable snd_SolidState is a Sound object, Sound doesn't have a stop method implemented; The Sound.play() however, returns a SoundChannel object which does...
So you can store that (the SoundChannel object returned by snd_SolidState.play()), which you can later use to stop the song.
PS: I recommend you start typing your variables (ie: var snd_SolidState:Sound;), that way you force yourself to know which object type you're dealing with and can find reference for it; in this case Sound.play().