Original patch was by dpiquet, I rewrote it and added some style/code things.
Use a slider to control the sound volume.
Currently the upper bound of sound gain is 10.0: http://trac.wildfiregames.com/wiki/Manual_Settings. But 1.0 is normal sound gain (not increased, not decreased), so upper bound would be good in [1.0, 2.0], because user with normal settings will have a problem to make gain in [0.5, 1.0], also the default gain value is 0.5. So I've set it to 2.0.
Also @echotangoecho has a good point about titling:
While you're at it, IMO, the "Gain" part of the sound settings names should be renamed "volume".
So I suggest to discuss and add it in the next commit, if all would agree.
Also I plan to add tooltip if it'd needed and support discrete values, like pass an array of string/float/int to it.