What is this device?

This was inside the bill of a baseball cap with the word "MARINES" on the front. I tried changing the batteries, but it doesn't seem to be doing anything. Does anyone know what this is?

It has a button on one side. But new batteries didn't reveal it's purpose. I'm thinking it's too corroded now to work. I know that round thing is a simple speaker or buzzer, but I don't know (a) what sound it's intended to make and (b) why it was sewn into the bill of a baseball cap.

Internals, image 1:

Internals, image 2:

External, backside:

Cap pictures:

Edit: added images of the internals.

Edit 2: added a small description of the button, and why I didn't just test it with new batteries (I did, but it doesn't work). So, basically, all I know.

Edit 3: added a picture of the backside of the casing.

Edit 4: Adding pictures of the cap itself.

Thank you, everyone, for all of your input. If I'm ever able to get this thing working, I'll add a video of the device and it's audio.