Soundtracks (to me at least) are a very interesting thing to discuss.
Depending on the question, they are more towards the music trivia aspect of the spectrum and obviously would not belong here. But they can be on-topic, when talking about the effect the sounds have on the movie.
Let's take a look at the questions we currently have tagged as soundtrack, shall we?
Does Dread have an official soundtrack?Does Dread have an official soundtrack?
This one I feel can be a bit trivia-like as it is purely a yes/no question and mostly the only reason it hasn't been closed yet is because it was asked when the site first came about and we didn't have the current moderators at that point.
This one has the most promise as it is asking about a deviation of a composer into a vastly different style. But it's greatest strength, is also its greatest weakness as that in itself is a music trivia about a score in a movie. However, if people are in agreement, we will keep this one as it does open up possibilities about famous movie-only composers/musicians known only for their movie performances.
Sci-fi movie that may have never existedSci-fi movie that may have never existed
This one isn't really about a soundtrack and is just using a music piece to help identify something so it does not actually count for this tag.
What is the theme song for House of Cards?What is the theme song for House of Cards?
This one is a bit of a curious case to me as well as it is asking a bit of trivia, but one could argue that the main theme song for a piece is very important to know and understand why it was chosen as such. While this question is not asking why but what it gets borderline for that reason alone, but can be a great example of a soundtrack question if formatted that way.
A good example of a sound related question asking a general question about movie/tv sounds in general and why they are put in when they are put in. An important piece of knowledge to understand editing techniques and proper production values.
Coming back to the same argument as the Dread question as it is basically asking what songs are in it and that's it. Borderline on these types, honestly.
So in general, this tag can have potential, and it can have pitfalls. Unfortunately, I cannot think of a proper way to make sure your question doesn't just turn into a piece of music trivia. Even though you're asking for sales, it doesn't really bring in much of an appreciation for the movies just because their soundtracks sold well nor does it really accomplish anything other than pure trivia.
I do appreciate you coming here to discuss this, however; and someone else may have a better idea for you, but I, do not.