First of all, the reasons artists are signing to labels are because of several reasons. For instance, if you (an artist) want to work with a well-known producer then you need to pay them immediately for their work which can range from tens of thousands to hundreds of thousands of dollars – which is before often generally taking a cut from sales of records too. Additionally, they also have to pay for the use of recording studios (even if the producer owns it), backing singers, special amp/FX required during the recording as well as, paying for the entourage of the producer to get to the recording studio etc. Most artists don’t have tens to hundreds of thousands of dollars to immediately for this hence, they sign to a record label that can ‘front’ the money for doing so.

Even if startups were disrupting this space - and there are some - the fact that artists have to pay producers up front etc is a challenge for them to overcome and record labels at the moment are a very good solution for this.

I mean even Jamal Edwards (the founder of SB:TV – http://www.sbtv.com – he's used as part of Google Chrome advertising in the UK - http://www.youtube.com/watch?v=0cKpvwlSGOI and even he has launched a Record Label albeit with Sony - http://www.rwdmag.com/2011/02/sb-tv-founder-jamal-edwards-si... and he was kind of disrupting the music industry by promoting artists that you may previously never of heard of.
(For instance, I first heard about Ed Sheeran - before he became famous and gone on to release numerous hits - through SB:TV's YouTube Channel)

Some music startups specifically covering what you said in the text area are the likes of:

https://www.webceleb.com/

http://www.nimbit.com/

http://soundcloud.com/ - Although Music Labels are often on there contacting users.

MegaUpload were planning on launching MegaBox/MegaKey before, all the stuff with them happened as well - which was to help unsigned artists/artists sell their music and keep 90% of earnings and to pay them for FREE downloads.

There are a few others as well but, I can't really remember them off the top of my head.

However, with regards to "disrupting the music industry" other interesting music startups to make a note of - and I’ve probably missed a load out since I’m just recalling them from what I can remember - as well as the obvious choices of Rdio, Grooveshark, MOG, Spotify, Pandora, Shazam and Last.fm (can they still be called a startup?!) etc and the ones listed above include:

http://www.earbits.com

http://www.mixlr.com

http://www.like.fm

http://www.deezer.com/

http://www.audiovroom.com

http://www.soundtrckr.com

http://www.songkick.com

http://www.thesixtyone.com

http://www.22tracks.com

http://www.watzatsong.com

http://www.macjams.com

http://www.stereomood.com

http://www.tunesbag.com

http://www.ex.fm

http://www.ejamming.com

http://www.mixcloud.com

http://www.thounds.com

http://www.gigjunkie.net

http://www.ujam.com

http://www.citysounds.fm

http://www.mugasha.com

http://www.mspot.com

http://www.bandzoogle.com

http://www.playgroundsessions.com

http://www.tastebuds.fm

There are tons more but these are just a few I can remember off the top of my head with regards to disrupting the music industry as a whole, and with regards to your specific question.