Recently, I watch some animations and the music faded in the end of it. How to make it ?

Audacity.
There's also an edit button below the properties if you click the frame with sound.

One way you can is after you add your music to your animation, click any frame your music is on open up your properties. Click effect: which should say None and select fade out. It will fade the actual ending of the song. But if you want to fade out early cause you didn't animate to the full song click the edit on the right of effect: and customize your fade out at will.

But yah audacity works
And fl studios (fruity loops)
Even camtasia studios

For Camtasia to fade you just drag a song into white space provided after opening then right click the song icon, click add to timeline at playhead. Right click the song in the timeline select edit audio, from there you can fade in, fade out, etc.. to custom fade right click song in the timeline, select add audio points it should mark your song with circle. Make another audio point and drag second one down for a fade. Hope it helps. Camtasia is a free download.

Thx dude, Really helps me a lot