I have a iPhone 4s 64GB for 3 1/2 years now. A few weeks ago, my volume bar randomly wasn't showing up when I tried pressing the + or - button on the phone itself.
Also, the volume bar was "grey texted" when opening the music app and it was completely gone opening the screen where you swipe from down the screen.
The problem fixed itself when I had the phone charged over 30% battery power and restarted the phone a few times.
When i'm getting a phone call or setting the timer, the speaker just works, no problems.
When I have a headphone plugged in, the sound also works normally.
When setting the option on to that i can set the calling sound with my volume buttons, the volume buttons work fine.
How does this come and how can i solve this problem??