Lower volume when another audio source plays

I have two audio sources:

• Audio source 1: Plays the background music.

• Audio source 2: Plays some guiding speech OnTriggerEnter on a game object.

I want to lower source 1's volume just before source 2 plays and then return the volume back to normal when source 2 is done playing. Lowering the sound is easy but getting it to return back to normal at the right time is a problem. Here is what I've tried

Player.js:

 var one_bg_sound : AudioSource;
 var two_guide_sound : AudioSource;
 
 function OnTriggerEnter (other : Collider) {
 
     // Lower background sound on collision
     one_bg_sound.volume = 0.25;
     
     // Play 4 seconds long clip
     two_guide_sound.Play();
     
     // After 4 seconds return the background sound to normal
     Camera.ResetBgAudioInSeconds( one_bg_sound, 4 );
 
 }

Camera.js:

 static function ResetBgAudioInSeconds( audioSource: AudioSource, secs: int ) {
 
     Debug.Log( "Reset music volumn in seconds:" + secs );
     yield WaitForSeconds ( secs );
     
     Debug.Log( "Music volume reset" );
     audioSource.volume = 1;
 }

The problem is: The ResetBgAudioInSeconds doesn't execute unless I remove the "yield" line. What am I doing wrong?