I have a brand new website for easy sharing of music. The files are intended to be copyright free (home recordings, podcasts etc). The system though could easily be used for music which does have copyright on it.
My intention is to become licenced by PRS for Music or similar service. This would make transfer of copywritten music legal just in case some are uploaded. This would be a safety precaution.
Based on my research a licence allows a specific number of tracks to be download -- for example it may allow 2,000 copys of any track to be downloaded.
Let's assume that I end up having 10,000 downloads. Who would be responsible for working out what exactly has copyright on it? Me? Them? The copyright holders who want money?
I dont want to find myself a few years down the line being forced to check through thousands of music sets to find specific tracks (next to impossible). Who is responsable for tracking the copyright issues?
If you do attempt to get approval first, you'll have to jump through a lot of expensive hoops. But yes, it is your legal responsibility to do just that. I am not advocating this path.
From my experience in the music industry, which is limited, you'll be notified by a lawyer that you are in violation as soon as they find out and if they are interested in collecting money from you. I am not advocating this path.
Which course you take will really depend on the music you plan to distribute as to what the risk is. If you are talking about making Michael Jackson's albums available for free, you'll probably run into legal trouble quickly.
If you want to distribute my non-label recorded album, you probably won't run into legal trouble. But if I contact you and ask what's going on, be ready to address me with an agreement and backpayment. That would make me happy. You could also have a web page online with statistics of artists that you are trying to contact.
If you really want to make an effort to contact someone, definitely check with ASCAP and BMI. You can search through their database for the people who own the music you want to use. If you can't find them there, odds are they won't be easy to find.
I'd suggest that you look up old Napster articles for your first thoughts of what can/could/would go wrong when you get involved with the music sharing industry. While it sounds like your idea is different this could get covered by Murphies law (if it can go wrong, it will).
It might not really matter who is responsible if you come under a flurry of litigation from the music industry hell bent on sending you straight back to lawyer hell unless you have very deep pockets to fend it off.
But to answer your question, from my understanding on it, all parties are responsible for it. As a webmaster, if you're in USA, you'll need a DMCA policies/methods set up to handle possible copyright infringements in a timely fashion (48 hours I suspect is the acceptable turnaround) on your site. The user who uploaded the content is ultimately responsible, but you are likely to get drawn in on the legal fight if it happens. And lastly the copyright owner is also responsible for finding your infringements and trying to sue you into the ground and get triple damages etc.
If you really want to continue down this path, I highly recommend that you hire a big law firm to protect yourself instead of relying on free internet advice.
I'd also suggest that if you're going into the music industry stuff, that you make sure you get a big stream of income to come with it to make the risk worth while.