I used a program called CAD 3-D by Expert Software. It is not made for speakers or even close to it but I use it to kind of get a general idea of what I want to build. The program has a grid so I use one line = 1 foot and the scale works out pretty good. It is an inexpensive program but it does the job for me. Just my 2 cents.
i have autocad 14 (i think that is waht it is) on my computer and it isnt to bad to use once you get used to it. it takes a while to learn and it is expensive if you can find guick cad id get it