Context mode Soundtrack uses all your available Music Packs as a replacement soundtrack for Elite: Dangerous. All you need do is turn it on by saying “context mode on”. Note that can you can switch between context mode soundtrack and context mode playlists, the latter refers to triggering your own playlists in game. Please see the article that discusses how to do that if you wish to trigger your own playlists.
“context mode soundtrack” command switches you to music pack soundtrack replacement mode
“context mode playlists” command switches you to triggering of your own playlists (see article about this for more info).