I help out a exhibitor at Capital Audiofest and I always play certain digital tracks. No. 1 on my list is Vivaldi Concerti pour Hautbois, Basson et Cordes with Alfredo Bernardini (Astree label)--I usually put on the 4th track which is a bassoon concerto.
Someone above mentioned the Be Good Tanyas. I also use one of their tracks off an album named Chinatown--"Waiting Around to Die" (a cover of a magnificent Townes Van Zandt song).
Another good demo track is the Natalie Merchant, Hazmat Modine, Fairfield Four performance of "Peppery Man."
For Jazz, I use Stefano Battaglia "Re Pasolini" album (either of the first two tracks), and Anour Brahem's "Thimar" album (jazz with an oud added to the mix).