How can I adjust my program's volume without changing the System Volume?

I want to create a program which is able to play background music when the desktop (the one showing icons) is focused.I have successfully worked out how to detect whether the desktop is focused or not, but when I use the PlaySound api to play music, I found it hard to adjust the volume. To make "fade in and fade out" possible, I have to adjust the volume of my program, and I need help with it.