Most likely, you're dealing with some sort of software issue.

Experiencing issues with the volume buttons on your Android phone? If it’s software related, it may be tied to a bug with one of your phone's accessibility features. Google posted on its Android ...

So you’re Android phone’s built-in speaker to listen to music while you eat your breakfast, and as you head out the door you pop in some headphones so you can keep the music going during your commute ...