You sure your program doesn't allow you to adjust the volume of the sound itself, and then the mixer track after that? If it is not letting you do so, then it may be the sampler you are using (although I still find it strange because FL simply has a volume knob for every last thing that creates sound, and then you send those sounds to a track, which has it's own volume knob, and I always thought this was normal and that every basic or more than basic audio program out there did). I can't imagine a program that would be like that, it would drive me crazy. Anyway, yeah the idea of the contest is for things to be far from standard and I personally wouldn't have any fun if it were simple and easy, so I'm sure you are not the only one who is having trouble, I think we all are, and it's how we cope with it that determines how well things come out. It's one of the best parts of this contest. Seeing how others coped with the difficulty.
Anyway, Dave, I think it's time you updated your software, or find out if there's more to it that meets the eye.
Edit: also, the order of how things works for when I make audio is, the mixer tracks are for volume adjusting (mixing), and the sounds themselves are used for dynamics if you adjust those volumes, If you do it in reverse it doesn't work and will seem limited. Also, you send dynamics (like distortion, vocoder, reverb...) to the mixer tracks, and then they each have their own adjustments (including amount of applied effect). (sorry if I'm stating the obvious, but I'm still confused as to how you can't do what seems like a basic function to me).