I remember when I first bought the vehicle the horn sounded different. It sounded less defined. I believe this is how the dual note horns work. Which my truck said it had on its build sheet. Now it sounds more defined but only in one tone. It is not as loud as it used to be either. Any ideas on how I can fix this?

 

Have you physically checked that you do have 2 horns.
If so disconnect one horn at a time then hit the horn. If both horns work you know they are both good. If one doesn't sound then that is the bad one.

It is also possible that one horn may have a slight tear in the diaphragm causing it to make a less pronounced sound.

 

Where is the diaphragm?

 

The driver side low note horn works. It's the passenger side high note horn that does not work. I have to remove the the air filter box to get to the horn. How do I remove it? I see there is some kind of retaining pin holding it in place on the front side of the vehicle. Do I squeeze it with pliers to get it out or what?

 

Horns used to have an adjustment screw that controlled the internal gap and if the gap got dirty, the horn quit. If there's still a screw, try backing it out a half turn and then back in. If it starts to work, you may have to tweak the screw to get the correct sound.