Instantiating guiText / guiTexture

Further to my previous question about showing/hiding guiText, i'm working on a closed caption system where a caption will show when the player comes within range of a sound. I'm using a guiTexture to create the black background, and guiText for the caption String.

Most sounds in the environment are continuous, so they should show a caption all the while the player is in range, then hide the caption if not. However, a couple of sounds are one-shot played by animation events so I want these to show for a period of time specified by me.

I previously got a system working that would hide captions after a specified time, but as described above, this isn't exactly what I want. In trying to amend my code to hide captions if the player moves out of range, I've discovered that now only the one-shot events seem to show up reliably. I believe the reason being is that i'm using one guiText and guiTexture for the one-shot events, and another pair for the general sound effects. Since the general sound effects call the function from Update(), I think they are overriding each other.

Due to this problem, i'm not even sure if my revised IF statement is doing what I want it to now. Also, I'm thinking, rather than creating pairs of guiText/guiTexture for each caption, a better approach is to instantiate them. I know that they will require screen co-ordinates (in my case i'm using x 0.5 and y 0.2), but how would I use this with Object.Instantiate?

Here is my current code:

function showCaption () {

 // if this sound source is in range of the player
 if(inRange){
     // if we are not already showing a caption
     if(!textOn){
         myCaptionObj.guiText.text = myMessage;
         captionBox.guiTexture.enabled = true;
         textOn = true;

         // if this is a one-shot caption, show it for the specified time
         if(!isLooped){
             if(myDuration > 0){
                 yield WaitForSeconds(myDuration);
                 myCaptionObj.guiText.text = "";
                 captionBox.guiTexture.enabled = false;
                 yield WaitForSeconds(myDelay);
                 textOn = false;
             }
         } 

     }

 } else {

     // if we are out of range, clear the caption and hide the box
     myCaptionObj.guiText.text = "";
     captionBox.guiTexture.enabled = false;
 }