I use FCPX and if you select the part of the clip you want slow mo with the range tool (R) and change the speed it will automatically apply a speed ramp, you can drag the ramp out or in depending how long you want it to ramp into slow mo. once you do that, and you go into the same tool to slow down the clip and at the bottom it has a setting for "preserve pitch" which you'll want to turn off. this will give you the ramp slow mo w the epic audio.
Probably are better more precise ways of doing this technique but this method works for my needs