I like to add the three different sound types (Music / Ambience / Sound Effects) to my text for different reasons. Here is how I think about each sound layer:
Music - Adds emotion and atmosphere to the text. If the emotion of the music doesn't match what the story is conveying the reader will have problems understanding the tone of your story. When choosing a music cue I try to find something that is subtle in its dynamics and not too 'in your face'. Also I look for something that fits the vibe of the text. For example an action scene would need something driving and determined sounding (like you would hear in a high action movie like 'transformers') a horror scene would need something slow and eerie and tense.
Ambience - Instantly lets the reader know where the scene is set. This works great for nature scenes for examples forests, beaches, suburban street etc. Try to pick ambience where individual sounds aren't too obvious. For instance, if you had a party scene, you wouldn't want to pick an ambience where an individual's voice / conversation is too easily heard as it would distract the reader from the text. Try to mix ambience at a medium volume that blends well with the music.
Sound Effects - Enhance the actions happening within the text. These can be nice and subtle for example: writing with a pen, sitting on a couch, footsteps walking, or be intense for example explosions, earthquakes, doors slamming. My best advice for sound effects is to use what works. If a sound isn't convincing or obvious I would leave it out rather than have something that could be distracting. It is also beneficial to take your time to find the right sound effect to fit the environment your scene is set in. For example, footsteps in a cave or large stone room would sound different to footsteps in a house / room. Searching for words like "Reverb", "Echo" or "distant" can help find sounds that feel like they are further away that can be helpful.
I love reading users Booktracks and helping them maximize their soundtracks potential, so if you have any questions / just want some advice on how to improve your booktrack don't hesitate to email me at firstname.lastname@example.org