Audio Tagging

You could define the global Sfx, Music and Dialog volumes in static variables in the volume control script, and have a special script attached to every object that has an AudioSource, which would define the type and the volume of the sound - something like this:

1- In the volume control script (let's call it VolumeControl.js):

static var sfxVolume: float = 0.7; static var musicVolume: float = 0.3; static var dialogVolume: float = 0.8;

function OnGUI(){ // adjust the volume of each type }

2- Attach the script below to objects that have an AudioSource:

enum SoundType {Sfx, Music, Dialog} var sType: SoundType = SoundType.Sfx; // define the sound type in the Inspector

private var iniVol: float; private var gVol: float;

function Start(){ iniVol = audio.volume; // save the local volume }

function Update(){ switch (sType){ // get the correct type volume case SoundType.Sfx: gVol = VolumeControl.sfxVolume; break; case SoundType.Music: gVol = VolumeControl.musicVolume; break; case SoundType.Dialog: gVol = VolumeControl.dialogVolume; break; } audio.volume = iniVol * gVol; }

I currently have a project that deals with many AudioSources. While I currently only have one volume level, the system I use could easily be extended to fit your purposes.

The volume control script (called VolumeControl.js):

 // Volumes stored into array as Sfx, Music, and Dialog indexed respectively
 static var volumes : float[] = new float[3];
 // audioSources stores a reference to each audio source in an ArrayList
 // based on what volume level is controlling it (parallel array with volumes[])
 static var audioSources : ArrayList[3] = new ArrayList[3];

 public static function SetVolume(volumeGroup : int, volume : float) {
     if (volume != volumes[volumeGroup]) {
         volumes[volumeGroup] = volume;
         updateVolume(volumeGroup);
     }
 }

 //Send the new volume level to the audio sources
 static function updateVolume(volumeGroup : int) {
     for (var i = 0; i < audioSources[volumeGroup].Count; i++) {
         var obj : GameObject = keys[i];
         obj.BroadcastMessage("UpdateVolume", volumes[volumeGroup], SendMessageOptions.DontRequireReceiver);
     }
 }

 public static function AddSource(audioSource : GameObject, volumeGroup : int) {
     if (audioSources[volumeGroup] == null)
         audioSources[volumeGroup] = new ArrayList();
     audioSources[volumeGroup].Add(audioSource);
 }

 public static function GetVolume(volumeGroup : int) : float {
     return volumes[volumeGroup];
 }

That script might need some modifications to better suite your needs but it should work out. You will need to add a function that removes the audio sources from the ArrayList if you do delete the AudioSources.

Then this script goes into the audio sources themselves. (Borrowing an 2 ideas from aldonaletto's answer.)

 enum SoundGroup {Sfx, Music, Dialog}
 var sGroup : SoundGroup = SoundGroup.Sfx;
 
 private var startVol : float;
 private var groupVol : float;
 
 function Start() {
     startVol = audio.volume;
     switch (sGroup) {
         case SoundGroup.Sfx:
             groupVolume = VolumeControl.GetVolume(0);
             VolumeControl.AddSource(gameObject, 0);
             break;
         case SoundGroup.Music:
             groupVolume = VolumeControl.GetVolume(1);
             VolumeControl.AddSource(gameObject, 1);
             break;
         case SoundGroup.Dialog:
             groupVolume = VolumeControl.GetVolume(2);
             VolumeControl.AddSource(gameObject, 2);
             break;
     }
     audio.volume = groupVolume * startVolume;
 }
 
 function UpdateVolume(volume : float) {
     groupVolume = volume;
     audio.volume = groupVolume * startVolume;
 }

These scripts will only cause slow downs when the volume changes and there are large amounts of AudioSources in the group being updated. All you need to do is call VolumeControl.SetVolume when the user touches the volume control. The system automatically checks to see if the volume level has changed and only updates if it has, so it is safe to call VolumeControl.SetVolume every frame, within the OnGui() function for instance.