How to STOP looping sound when going into next frame/Errors

I have a flash project broken up into multiple frames, with a button on each frame that goes to play the next frame. (And a movieclip on each frame that plays until you hit next frame button)

On each frame, I want audio to play, and loop. But, I want the audio from one frame to stop when I click the button to go to the next.

On frame 4, I have this code:

import flash.media.SoundChannel;

var sound:Sound = new firt2();
var soundChannel:SoundChannel;

sound.addEventListener(Event.COMPLETE, onSoundLoadComplete);

sound.play();

function onSoundLoadComplete(e:Event):void{
    sound.removeEventListener(Event.COMPLETE, onSoundLoadComplete);
    soundChannel = sound.play();
    soundChannel.addEventListener(Event.SOUND_COMPLETE, onSoundChannelSoundComplete);
}

function onSoundChannelSoundComplete(e:Event):void{
    e.currentTarget.removeEventListener(Event.SOUND_COMPLETE, onSoundChannelSoundComplete);

}

And it works. However, I want to stop it once I click the button to go to the next frame. I have tried: soundChannel.stop(); On the next frame.

However, whenever I do that, the output reads:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at hhh4_fla::MainTimeline/frame5()
at flash.display::MovieClip/gotoAndPlay()
at hhh4_fla::MainTimeline/fl_ClickToGoToAndPlayFromFrame()

All of my buttons and movieclip have instance names.

Rather than figuring why it doesn't work with all these frames and timelines, I think it's better to compose a centralized sound manager class that handles these things.

Implementation. Keep in mind that I didn't test that so please excuse me for occasional typo if any. The logic of it all should be correct.

package
{
    import flash.system.ApplicationDomain;

    import flash.media.SoundChannel;
    import flash.media.Sound;

    import flash.events.Event;

    public class Audio
    {
        // Container to cache Sound objects.
        static private const cache:Object = new Object;

        // Variables to hold the current values.
        static private var currentChannel:SoundChannel;
        static private var currentSound:String;

        // Stops the current sound playing. If you pass the sound name, it
        // will stop the audio track only if it is the exact one playing.
        // Otherwise it will stop any one currently playing.
        static public function stop(value:String = null):void
        {
            // Do nothing if nothing is playing right now,
            // or if the specific sound requested to stop does not match.
            if (currentSound == null) return;
            if (value) if (value != currentSound) return;

            // Unsubscribe from event and stop the audio.
            currentChannel.removeEventListener(Event.SOUND_COMPLETE, onComplete);
            currentChannel.stop();

            // Final clean-up.
            currentChannel = null;
            currentSound = null;
        }

        // Plays the embedded sound by its class name.
        static public function play(value:String):void
        {
            // Do nothing if the requested sound is already playing.
            if (value == currentSound) return;

            // Stop the current audio track playing.
            stop();

            // Check if that one sound is valid and/or was previously requested.
            if (!cache[value])
            {
                try
                {
                    // Obtain class definition from the project.
                    var aClass:Class = ApplicationDomain.currentDomain.getDefinition(value) as Class;

                    // Try instantiating the Sound.
                    if (aClass) cache[value] = new aClass as Sound;
                }
                catch (fail:Error)
                {
                    // Well, do nothing, yet.
                }
            }

            if (cache[value])
            {
                // Store the id of audio track that is going to be playing.
                currentSound = value;

                // Play the track and subscribe to it for the SOUND_COMPLETE event.
                currentChannel = (cache[value] as Sound).play();
                currentChannel.addEventListener(Event.SOUND_COMPLETE, onComplete);
            }
            else
            {
                // If there's no such class, or it is not a Sound,
                trace("ERROR: there's no sound <<" + value + ">> is embedded into the project.");
            }
        }

        // Event handler to clean up once the current audio track is complete.
        static private function onComplete(e:Event):void
        {
            // Sanity check.
            if (e.target != currentChannel) return;

            stop();
        }
    }
}

Usage.

import Audio;

// Any time you want different sound to play.
// Pass the class name as Sting as an argument.
Audio.play("firt2");

// Any time you just want to stop the sound;
Audio.stop();