I'm not an expert on licensing, but I looked into it before, and you can obtain both mechanical and digital licenses pretty easily for many songs at Harry Fox Songfile. If the songs aren't there, the process is more involved, but still doable. Digital or mechanical, I think the standard statutory rate of 9.1 cents / song applies to each download or physical disc.
Our CD contained all original songs so we didn't have to worry about licensing. Our music is folk rock / indie rock so check it out if you're interested.
Lightning Inside You