Check with the software to see how many threads it can run at one time. With the Intel® Core™ i7-3930K you are going to get a 6 core processor with support for up to 12 threads. With the Intel Core i7-3820 you will get a quad core processor with up to 8 threads of support. These benchmarks may give you a better idea of how they would stand up against each other http://www.anandtech.com/bench/Product/523?vs=552
In the end it comes down to the fact if your software will use the extra threads and how much of a boost will you get from it.
I wouldn't be so sure. Music can sometimes be more intensive than Video rendering. I agree with the Intel guy, check your software (or call the software maker) and see if they can use 12 threads at a time. If not, then you should be great with the quad core.
If you can use unlimited amounts of threads, you may even wish to consider a dual processor XEON setup with dual 4 or 6 core processors.