As most people here have pointed out already, raw-data is crucial for an economics ee. Raw data means that you need figures (from the companies in question) which you will process. My theory is though that you probably cant obtain information from all cd selling companies, so you need to narrow dont your question even more. You have to tell how SOME firms have been affected by the introduction of mp3. That is, your research has to be more focused on micro than macro economics.
The above is your base. In your essay you naturally need to state your purpose and research question, how youve narrowed it down, your sources and method. Then you need to include relevant theoris, as you mentioned yourself, for instance, theories on price and demand, elasticity, etc. When youve done this, youll have to go ahead with your analysis. Include A LOT of diagrammes. Make sure theyre clear, well labelled and all that stuff. Very important! If youve detected some deviations from your theories, MENTION it! IB loves critical thinkers. Then you conclude. Oh, and its a good thing of you put all your raw-data in an appendix. (Dont confuse this with your processed data that you will include in your essay.)