How about this? Small, sturdy, quiet, cool, energy efficient, performing greatly, very easy to build, very easy to carry around without detaching any hardware, looks good overall.
No discrete GPU (since you'll be doing sound recording/editing and image/video editing, CPU's integrated video would be more than enough. And you could add GPU later anyway, if you feel like doing so), no Disc Drive (You can install Windows from a Flash Dongle, and all needed drivers you can always download online/install from a Flash Dongle. And even if not, you can add Disc Drive later if you feel like you need it, which is highly unlikely), no OS included (read previous sentences. You pretty much have it on Flash Dongle/CD already if you're preparing to do such a build). Also, no peripherals (like keyboard/mouse and etc., I'm just assuming that you have them already, so I didn't count those).
Man...just get exactly what I've picked to you and you'll be absolutely fine. The motherboard that I offered is the best ITX board so far, there's just no better.
It is a very niche market these days, even though it's growing little by little every day, but there's still not much truly worthwhile products in this segment up to this day.
Everything that I've picked for you in the first post, is the best you could get for such money, so far. If you'll try getting some other board, you'll be just getting worse product.
It's a very balanced (high quality, high productivity, very comfortable, but at the same time meets your proposed budget completely) build, just exactly what you need out of ITX.