Controlling A Single Sound

Hi. I have created a CD ROM and am trying to iron out this glitch with my sound. I have a sound clip (trains.wav) that is activated when I play a small animation. I have a Stop button and a play button for this sound. This animation and sound is contained in a movie clip where i have created the variables sound on and sound off on a layer. For each button i call on these variables to turn the sound to 0 or to 100. e.g:

layer code: var soundOff = new Sound(trains.wav);

button code: on(Release){
soundOff.setVolume(0)
}

This all works as expected. The problem is that I specify that the single sound is turned off as seen above, but when i go to play a video on a different movie clip, the sound is switched off here as well, and cant be turned on till i go back to the animation and click the sound on button? I really need help on code to toggle this one sound!