up-volume button has lost its "spring" and "click"

Volume switch flex cable.

@petehy , Peter the 1st link is a guide to show how to disassemble the device to check for possibly dust/debri/gunk or fault, which could be the headphone jack/volume control cable available( I believe what Tom is also referring to) for purchase at the 2nd link. Good luck.

I hope this helped you out, if so let me know by pressing the helpful button.

iPhone 4S Headphone Jack Cable Replacement

[연계 제품이 누락 또는 비활성화 됨: IF115-013]

This is what the actual buttons look like on the inside (to the left in the image):

http://applepartsstore.com/media/catalog...

The yellow plastic covers with a tiny black "tit" in the middle of is the only thing you need to replace to resolve your issue. It's way easier than replacing the whole headphone jack part, and it shouldn't take you more than 5-10 minutes if you have experience in repairing phones. What's happened on your phone is that the tiny black "tit" in the middle of the yellow plastic has been worn out or, more probably, completely loosened from the yellow plastic. Underneath the yellow plastic there are two identical metal plates stacked on top of eachother, and it's very important to make sure that there's still two of them there when you replace the yellow plastic part.

You simply order a headphone jack for an iPhone 4 or 4s, it doesn't matter which one, and carefully peel off the yellow plastic part from your old volume-up button and discard it, and then carefully peel off the same part from the new headphone jack flex and move it to the headphone jack flex in your phone. Put some pressure on it with your fingers so that it sticks, and then put your phone back together.

Feel free to ask questions if any of this is unclear, I'm too restless to read through what I just wrote right now.