Why does the volume bar on my iphone disappear and appear whenever it wants to?

I have an iPhone 4. If I try and listen to music the volume bar goes away and when I try to adjust the volume with the + and - buttons the volume bar under "ringer" is gone. I've tried restarting my phone, fixing the settings, plugging in my earphones and cleaning where you plug them in and connecting my phone to my laptop but nothing seems to work.

Restoring your iphone may fix the problem because the problem you mentioned is due to some problem in iOS…

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