Function to fade audio smoothly from one level to another in AS2

Question

Greetings all.
I'm a newcomer to Actionscript programming and trying to modify a Flash site template downloaded from TemplateMonster.com (which is an AS2 template).  I've almost succeeded at what I need to do but have run into a couple of brick walls.  One is that I've added a video player to the site and need to make the background music track smoothly fade out when the video starts and fade back in when it ends (or is stopped).  I set up a listener object for the video player that works.  It's the smooth fade of audio levels that doesn't.
I found an excellent thread from last year (http://forums.adobe.com/message/3236495) in which kglad address the issue of fading audio from one level to another.  Since TemplateMonster's templates set up a master movie clip and then load pages to play within it, and I need to call the function from within the pages, I tried setting it up as a global function.
So when the overall site initializes, I have this:
this.createEmptyMovieClip("mcMusictrackHolder", this.getNextHighestDepth());
var sndAudio:Sound = new Sound(mcMusictrackHolder);
var nMaxMusicVolume:Number = new Number(mcMusictrackHolder);
nMaxMusicVolume = 30;
sndAudio.attachSound("MusicTrack");
sndAudio.setVolume(nMaxMusicVolume);
sndAudio.start(0,9999);
// Here is kglad's function converted to a global function
_global.fadeSoundF = function(mc:MovieClip,s:Sound,vol:Number,sec:Number):Void
     trace("Getting here with sound at " + s.getVolume());
     clearInterval(mc.fadeI);
     var volumeInc:Number = vol-s.getVolume()/(10*sec);
     mc.fadeI=setInterval(fadeF,100,mc,s,volumeInc,vol);
     trace("Leaving with sound at " + s.getVolume());
function fadeF(mc:MovieClip,s:Sound,inc:Number,endVol:Number):Void
     s.setVolume(s.getVolume()+inc);
     if(Math.abs(s.getVolume-endVol)<inc)
          clearInterval(mc.fadeI);
Then within the page that contains the video player (a child of the above), I have this:
var listenerObject:Object = new Object();
var sCurrentState:String;
listenerObject.stateChange = function(eventObject:Object):Void
sCurrentState = my_FLVPlybk.state;
if (sCurrentState == "playing")
  fadeSoundF(_root.mcMusictrackHolder,_root.sndAudio,0,1);
else
  fadeSoundF(_root.mcMusictrackHolder,_root.sndAudio,_root.nMaxMusicVolume,1);
my_FLVPlybk.addEventListener("stateChange", listenerObject);
The listener object is working fine - so if instead of calling fadeSoundF I simply do _root.sndAudio.setvolume(0) to mute and _root.sndAudio.setvolume(30) to restore it works fine except that the volume changes abruptly instead of ramping.
But obviously there's a problem with my effort to convert kglad's function to global and use it that way, because the trace statements tell me the audio is getting set to random levels.  It smoothly ramps all right - but tries to get to -472 or +8212 or other insane numbers.
Any help with where I'm going wrong would be deeply appreciated.  Keep in mind that although I have some long-ago programming experience in other languages, this environment is completely alien to me and you can feel free to assume I'm completely ignorant.  What I've managed to piece together is largely pulled from online research, which is why there's probably an obvious glaring error in there.  Feel free to provide a response that assumes I know nothing.
Best,
Pete

Answer 1

I added a few more traces.  Here's the new main page:
this.createEmptyMovieClip("mcMusictrackHolder", this.getNextHighestDepth());
var sndAudio:Sound = new Sound(mcMusictrackHolder);
var nMaxMusicVolume:Number = 30;
sndAudio.attachSound("MusicTrack");
sndAudio.setVolume(nMaxMusicVolume);
sndAudio.start(0,99999);
_global.fadeSoundF = function(mc:MovieClip,s:Sound,vol:Number,sec:Number):Void
          trace("Getting here with level at " + sndAudio.getVolume());
          trace("asking to set it to " + vol + " in " + sec + " second(s).");
          clearInterval(mc.fadeI);
          var volumeInc:Number = vol-s.getVolume()/(10*sec);
          mc.fadeI=setInterval(fadeF,100,mc,s,volumeInc,vol);
          trace("Leaving with sound at " + sndAudio.getVolume());
          trace(" ");
function fadeF(mc:MovieClip,s:Sound,inc:Number,endVol:Number):Void
          s.setVolume(s.getVolume()+inc);
          if(Math.abs(s.getVolume-endVol)<inc)
                    clearInterval(mc.fadeI);
Here's sample trace output:
Getting here with level at 30                  <---- this is on first visiting the page where the listener object initializes and asks it to ramp the audio from its
asking to set it to 30 in 1 second(s).             current level to the same level (probably because the video is buffering or rewinding and that counts as a state change).
Leaving with sound at 30
Getting here with level at 111                <---- this is a second triggering of the function (probably because the video is STOPPED and that counts as a
asking to set it to 30 in 1 second(s).            state change after buffering).  As of yet the video hasn't been asked to play.
Leaving with sound at 111
Getting here with level at 3765               <---- This is what happens when the video is asked to play; since it is asking to leave the level at 30 I'm guessing it is buffering
asking to set it to 30 in 1 second(s).              again but not playing yet, which is why it's still being asked to set the background to 30 instead of 0.
Leaving with sound at 3765
Getting here with level at 3765                <---- NOW the video is actually playing, so it asks to set the background audio to 0.
asking to set it to 0 in 1 second(s).
Leaving with sound at 3765
Getting here with level at -41740             <---- ... and I hit STOP, which generates a request to bring the background audio back to 30.
asking to set it to 30 in 1 second(s).
Leaving with sound at -41740
I should probably replace the if/else on the video player page with a switch/case so the function ONLY gets called when playback actually stops and starts, and doesn't get triggered for every state change.  But I think I still have an error in here someplace...
Many thanks to kglad for your help!
Pete