I have never used fleeper's approach. But, an alternative is to add an audio envelope to the music track (Insert menu, Audio Envelope->Volume). Then, you "add point" to the envelope by right clicking while the pointer is on the envelope line at the point you want to fade. You need to add multiple points. The first point anchors to the original volume. The second point is at the new volume level (move the point down to fade the music lower). The proximity of the points (in time) allows you to control how quickly the fade occurs. I use this method on my voice track a lot. It helps to increase the volume when someone says something that wasn't mic'd very well and allows me to lower the volume during periods where there's only background noise.

In looking back at fleeper's suggestion, I think it already assumes that an audio envelope exists. But, I could be mistaken.