Do you mean like positional sounds? (they get louder and quieter as you get closer to them), or pure volume fade?
If so, then the best way is to use a global variable for volume (eg: "MusVol"), maybe a second global variable for whether to fade in/out or not at all (eg: "MusFade", 0 for no fade, -1 for out, 1 for in), then a variable for how much to fade by per second (eg: "FadeSpeed").
Code would look something like this:
IF MusFade = -1 & MusVol > 0 THEN MusVol = MusVol - (FadeSpeed * timedelta)
IF MusFade = 0 THEN MusVol = 100% (I forget exactly if it's in a value between 0-1, or 0-100, or even in dB where 0 is loudest)
IF MusFade = 1 & MusVol < 100% THEN MusVol = MusVol + (FadeSpeed * timedelta)
That's in pseudocode (not real code of any language), so things like "MusFade" would have to be written in Construct's way like: global.value('MusFade')