The most simplistic way would be having two .wav samples, one louder than the other

This means of course that you need two separate SoundPlayers and decide which one to play.
Or you could load the needed sample for every metronome "beat" (Sorry, I'm musically impaired ) to the single SoundPlayer class.
I would go for the former solution.

I agree with BigEd though, don't fiddle with users volume settings unless its the purpose of your program.
If this program is for your own personal usage and you still want to do it, I suppose you could do some interop calls for winmm.dll. Have a look at http://www.dreamincode.net/forums/showtopic45693.htm
It is a rather long article, but it does cover a lot to do with volume and the mixer.