The Audio Sequence
The Audio section of the Sequence is divided into two areas, one for your video's audio and the other for separate audio files (e.g., music, sound effects).
The first section is called the "Clip Audio Track". This is a dedicated track for your video's audio. Audio files that are not attached to video cannot be placed on this track.
The second section is called the "SoundTrack" area. These tracks are for any music, narrations or sound effects that you would like to add to the sequence. There are an unlimited amount of soundtracks available, and you can create a new track by selecting an audio file on the sequence and "Right-Click > Move to track > New Track". You can then toggle between the tracks using the up down arrows to the left of the soundtrack sequences.
The Audio Palette
The Audio Palette is the area to the left of the Sequence timeline area that displays the name of the audio track (which can be changed from the default name by clicking that track's title bar), a volume adjustment slider and buttons for adding audio effects and muting. There is an audio palette for each audio track.
You can add audio effects to an audio track. Click the star button in the track's audio palette to open the "Effect Chain" dialog. In the Effect Chain dialog, click the "Add" button to add a new effect to your audio track. Use "Remove" button to delete audio effects from the list, or disable the effect by unchecking it. Click "OK" to apply chosen effects. Learn more about each effect in Audio Effects
You can add fading to your audio track by clicking "Edit->Sound->Fade in/Fade out" in the top context menu or just right-click on your audio track and select "Fade in" or "Fade out" and set fading duration in seconds.
The other way to work with fading is to create fading points on your track's waveform. With a audio clip selected, in the Clip Preview area, click on the yellow line in the middle of the waveform to create anchor points. You can then drag these points up and down any way you like. Up will increase the volume and down will decrease the volume.
To remove any particular fade point, right-click on it and select "Remove fade point". To remove all the fade points right-click on your audio track and select "Remove fade points".
How to disconnect native audio from video
Your clip's native audio comes connected to the video by default but you can change it. To unlink audio from a video clip, right-click on the audio and select "Unlink Sound Clip(s)".
You can also save a video clip's native audio as a separate file by right-clicking the video file in the Media List and choosing "Save audio track as a separate file" from the context menu.