YouTube is not the service provider when the video is accessed as an integrated part of the OP's website. That YouTube take action against infringement doesn't mean that you're not infringing by embedding copyright content available there on your own site (particularly as it's commercial).
http://www.out-law.com/page-7235, http://www.theregister.co.uk/2006/08/29/guitar_sites_under_f... give details of take downs, a few years ago, around the provision of guitar tabs. Note that the MPA moved the next year to make specific provision for licensing of guitar tabs, http://arstechnica.com/tech-policy/2007/04/legal-guitar-tabs....
I suspect Capo, http://supermegaultragroovy.com/products/Capo/, operates the way it appears to because they can circumvent many things (that copyright in the music or production of the tracks would restrict) by using the users purchased tracks as the source for the information.
Soundslice looks awesome but if they've not yet consulted with copyright experts then I fear they may be looking at a lawsuit imminently.
Choosing Yesterday [which is beautifully played] as one of the example tracks was pretty ballsy. FWIW the YouTube video links to Bandcamp where an album of covers are available - Bandcamp are obviously aware of this sort of issue, they can't sell unlicensed covers lawfully. Bandcamp T&C (http://bandcamp.com/faq#mixtape) specifically rule out the use of their service for covers. $0.091 is the current fee per digital play of the track (audio only), sync fees can be more or less; see eg Limelight or HFA or BMI.
http://diymusician.cdbaby.com/2012/03/on-posting-cover-songs... covers the licensing requirements quite well.