Assuming gaming at ultra high framerates is a low priority and you would value quietness, I would go with something like this:
Intel Core i5-2500k
8GB Mushkin Silverline 1333Mhz
XFX Core Edition HD 7770 1GB
Seagate Barracuda 500GB
Antec Sonata III 500 with 500W PSU (EA-500)
Coolermaster Hyper 212 Evo
Sony DVD Burner
Total - $820.92
This goes slightly over even the higher end of the budget but would be fantastic for the audio stuff while being extremely quiet due to the great case and low power consumption.
If gaming is more of a priority, you could get better bang for your buck. This is still a great gaming machine though, the HD 7770 is about as powerful as a GTX 460 1GB. I have the GTX 460 and can still play virtually anything on ultra high settings @ 1920x1200. I just have to tone down the AA and AF slightly on something like Skyrim or BF3.
The main reason for the aftermarket cooler was the quietness, also that motherboard has everything he needs and just happens to be Z68 so it would seem a shame to not spend that little bit extra on the huge overclocking potential of the i5-2500k (even if it's not straight away.)
The build that wasupmike posted is a good example of what you could get by sacrificing quietness for a bit more gaming performance.
the core i5 Sandy Bridge CPU's run pretty cool at stock settings even under high stress - the Intel box coolers will do the job fine, and even though they're not the best quality you can get.. they run quiet when not overclocking...