I am a web designer who was tasked with redesigning a website that sheet music, freebirdnotes.com. The new website has been live for about 3 months but the bounce rate is still very high. I've compared the website with other similar websites, but can't figure out why the bounce rate is so high.
Driving traffic is not your problem. Even the world's largest publishers of piano sheet music are struggling. They, and you, all share one problem: you aren't solving the problem people looking for sheet music have. You are all assuming the problem is "I need sheet music for x." Wrong. The problem is "I need to present a successful performance, wow my friends, please someone (even, maybe just myself), by being the 'bringer of joy' in a musical setting."
Getting the written instructions for playing the music is one small part of that job or need. You want to stand out? Turn your sheet music sales site into a place where more of what they're trying to accomplish can actually happen. Supplying the sheet music might become secondary in that setting, but unless you go in that direction, you'll be a distant competitor to 8notes.com, sheetmusicplus.com, imslp.org, sheetmusicdirect.com, amazon.com, and many, many others.
Think about it from the customer's perspective: I want X sheet music. Oh, great, there it is on Amazon, who I know and trust, and, boom, I have it. Wait, what? There's freebirdnotes.com, too? Never heard of it. They offer the same thing, but cheaper? Nah. I'll stick with Amazon because, well. Amazon. Wait, you say you've got good plano arrangements? How do I know they don't suck? Nah. I'll just stick with the professionally arranged versions by known publishers."
Also, there is a VERY high likelihood that your product is illegal. The right to create and sell, or even share, arrangements is something you would have to secure in advance from the publisher of the original musical work, a right which they most likely would deny to license to you. Delegating the arrangement and posting of content to "members" is probably scant protection for you, as your platform enables and monetizes creation and sale of these arrangements.
I'm not qualified to give you legal advice, but I can say with certainty that you should get some!