Are you sure you want to go into the legal profession? Have you already studied for a degree? A degree is more importnat than the A levels.
Some uni's will not consider an A levels after your first sitting. Some will. You have to call and ask- they may be lenient seeing as though you will be a mature student. If they will consider the extra A level - go for it.
I'm assuming you want to study Law (llb that gives exemption toCPE/GDL) at degree level as opposed to something else and then the compulsory CPE/GDL; before the LPC. This, and the specific uni's you are thinking to apply to will have a bearing on whether or not your music tech. subject will be a hinderance or not. After that, so long as your degree is a 2:1 +(2:2 for some frims- don't rely on this though) you will be fine.