More Thoughts
It's not a bad start for a system but here's a more thoughts.
1) Categorise the music by both category, media and title, that way you can keep a much closer eye on stock flow and then set up automatic trigger points for re-ordering. The tighter re-categoristaion of sales will also allow you to profile sales trends better, therefore you'll have much better control over stock, leading to lower overheads and if you have the system highlight trends you can react much quicker to those trends.
2) Input a marker for which clerk (I assume you'll have more than one), sold what items. Might as well if you ever need to track in month sales figures.
I can't anything fundamentally wrong with the system, it's just that it's a bit too simplistic for me to do much with I'm afraid and without knowing what the objectives are there's only so much I can suggest. Sorry.
If you want to send me the rest I'll have a look at it for you though.