Promising nothing less than near-immediate delivery of more than 8,000 pieces of sheet music, this Wisconsin-based vendor boasts a hassle-free plug-in that plays on the Web page itself. But what’s to play? The pop-rock offerings are dismayingly generic: multiple versions of Bryan Adams’ ”Everything I Do (I Do It For You)”? And no Beatles? Instead, browse the impressive, 200,000-strong paper inventory (available by mail order) until the future arrives.