In general, you want to deliver sounds at a fairly 'hot' level, and set the volumes in software.
But first find out how exactly the programmer sets volumes. Is it on a 0.0 - 1.0 scale? (common for Flash), is a MIDI-like 0-127? is it in dB? Can it "go to 11" (i.e. play a sound louder than it's default level).
Knowing how their system deals with volume is very important for you to remain in control of the mix, even indirectly.
You want to make sure that when you deliver the sound, you also deliver the volume level it should be. Otherwise, you'll have the programmer either 1) guessing or 2) just playing everything full volume.
So, for example, your delivery might a spreadsheet that looks like this
Event WaveFile Volume
Select SelFX3.mp3 0.7
DeSelect DeSel.mp3 0.6
mainMusic MyTheme.mp3 0.3
That way, the programmer knows what to set to what.
... or talk them into something like FMOD or Wwise that gives you the control
There are some exceptions-- if you have a very quiet wind rustling sound for example, it doesn't really make any sense to normalize (set to max volume) that sound.
Also, if you have a set of sounds that go together, it's good to have their levels be correct relative to each other.
Brian Schmidt Studios
Executive Director, GameSoundCon:
GameSoundCon 2016:September 27-28, Los Angeles, CA
Founder, Brian Schmidt Studios, LLC
Music Composition & Sound Design
Audio Technology Consultant