CMO, CPO, VP Product, CEO. Pitch, planning and presentation expert. Also can help with board/founder issues, GTM strategy, product planning/fit
You would have to find out who the publishers are for each and every title, and then write to them to ask them for permission to a) create a transcription, b) sell it in territory X, and c) charge money for it.
There is no middle man I am aware of that can get you those permissions and licenses en masse.
Publishers are under no obligation to grant you permission, so you are most likely to get a standard rejection from all of them. They would rather not create competing products that would deliver them less revenue than the transcriptions being sold by companies they have already licensed, like Hal Leonard.
Without written permission form ALL of the publishing rights holders (there can be multiple publishing rights for single titles), and without a valid license agreement, the only way to legally sell transcribed sheet music is to make sure the original compositions you are transcribing are in the public domain.
For certainty, seek legal advice from a music rights attorney.
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!
Your idea has too many masters to please. It's for the industry, but it's also for the artist. It presumably enables the artist to accomplish something with their fans/customers. So, it's for them, too. Pick one problem and solve it.
Don't bother trying to solve a problem for the big 3. They have only one goal when it comes to investing in tech companies: extract as large an up-front licensing fee as possible before the company runs out of cash. Consequently, they're looking for companies with a fresh seed or series A or B round whose business model absolutely relies on licensing their IP.
They are not interested in placing a bet on anything else. I know because I wasted a lot of time pitching to them (Sony, UMG, WMG).
You'll do much better focusing on solving a problem for the artist that doesn't make them replicate what they're already doing. In other words, you don't want to be YET ANOTHER middle man. Yet another platform for them to have to play on. They won't join unless you have massive, paying fanbase, which you don't ... or you can SHOW that your platform more accurately, intelligently, or otherwise successfully connects the artist with the specific people they want or need to reach.
That's a tall order, and like any platform play, it suffers from the chicken and egg question. How do you get a valuable group of users (fans), without the artists on board? And how do you get the artists on board without the fans? Simply setting up tools, favorable monetization structures and architectures won't be enough to attract either side.
But you asked about successfully pitching the idea. Short of building test version that can validate your assumptions, you could devise an architecture and set of rules (algorithms) that in some way makes artists more successful. If it comes across as plausible, you might be able to successfully pitch that. For example, you could say your system is more efficient, pulling together artist data and media from Bandzoogle, Reverbnation, Sonicbids, iTunes, Tidal, CDBaby, etc., and compiling it with their own social media accounts, event calendars, merch sales, etc., to produce a single dashboard that ... what? That's the key. In what way does it make the artists' lives better? Remember, they have almost zero incentive to try something new, untested, without an audience, even with "good" terms. Why? Because the amount of work required to set up on the new system isn't worth the risk. Unless you make it obvious that it is.
If you are able to create such an app idea, or place (even "on paper"), investors will want to know why you, personally, and your team are the best people to actually make it happen. If you can hit that note, too, then they'll want to know why you won't be made irrelevant if company X decides to simply add features you propose to their already vibrant businesses. Lastly, if you can convincingly and compellingly answer those questions, they'll want to know how much it costs, how long it will take, and how they will make their money back.
First, though - pick just one master to serve, and make it all about them. Doesn't matter if it's artists, fans, industry promotions folks—as long as it's clear your idea will make their jobs better (easier, cheaper, faster, more profitable). Ancillary opportunities can be glommed onto that core product later, if you succeed, but without focus, you risk appearing as a bad investment.
I've managed digital products (apps) from concept to maturity. My experience is with SaaS and apps, so it may or may not be relevant to your situation.
I think the best tool for prioritizing features is to create a features scorecard in a spreadsheet and use that tool to get alignment with your team as to what features get what priority.
Start with defining your business goals (e.g., get more revenue, increase word of mouth, reduce churn, increase retention, etc.). Then assess the resources you have (e.g., number of developers, designers, writers, customer support, sales, whatever your company has to "spend" to create any feature.)
In your spreadsheet, create a row for each potential feature and a column for each goal and each broad category of resource (engineering, marketing, support, for example). Use a small numeric scale (0 - 3) to "score" each feature in each column. For example, a feature may not move you toward goal #1, so you would put "0" in goal #1 column. For the resources, do the same. To the right of all these add a column that subtracts the sum of the resource or "effort" scores from the sum of all the goal column scores. You can multiply these scores by a confidence factor as well.
If you have a team, it's important to get their input on the scoring. This is not something you just show them and ask for approval. You have to involve them in a meaningful way. I can show you how to do that and point you to some examples you can use, if you'd like to give me a call.