Separate the audio from the video.
Then do the following:-
1. Click Create>Create Sound>SoundFX1
2. Choose the audio.
3. Click Track List>Project>SoundFX1 in the Timeline.
4. Click SoundFX1 tab.
5. The audio clip should be in the track.
6. Click Animation>Sound Modifier>SoundFX1
7. Modify the volume/ Fade in/ Fade out.
YouTube Tutorials Here