First of all, it's important to note the E5520 is intended to be a server processor. I'm thinking the i7-920 would be the better performing CPU for an actual workstation environment. Especially since it's $90 or so cheaper. Plus the i7-920 is going to have a higher stock speed since you're not wanting to overclock.
Unless the audio software you'll be using somehow operates better with a Xeon setup, I'd say the i7-920 is your best over all option.