It is a great question, and as dren says, it is very dependent on the platform.
Try to make sure that they give you a build of the game so that you can determine if *you* are satisfied with the levels and mixing of the game.
The actual method for setting the volume of the sounds will depend a lot on the process the developer uses to put sound in the game. It can range anywhere from “you have to pre-bake the volume into the wave file” to “you can set it on the fly as you’re playing the game via audio middleware like FMOD/Wwise, etc.”
Do not that on mobile, developers generally like things fairly ‘hot’, because a lot of devices don’t drive a ton of volume to the earphones.