Public Domain
Older songs sometimes will fall under the public domain catagory. I had to look up because I forgot the timespans(been a while since my music business class).
If it was published before 1923, you are all clear
1923-1963 If the copyright did not pass on from the original owner to a child/spose, or owner just plain forgot to renew copyright, you are safe.
It's overall a big mess of dates
http://www.unc.edu/~unclng/public-d.htm
http://en.wikipedia.org/wiki/Public_domain is a little less nifty a layout, but it goes a little more indepth
That's all your public domain information, found my copy of the copyright act and as of 2002 this is current, I don't believe there have been any changes in that portion of public domain part. You may want to double check this, I'll look a little more into it as well and let you know if I find it has changed
If it does not fall into public domain
http://www.cleverjoe.com/articles/music_copyright_law.html
This gives a pretty nice rundown of how to obtain rights for a cover
Protecting your property
Next order of business, protecting your recordings. You could do the old mail yourself a copy and don't open it ever unless you need to in court, in front of the people who would need to verify it has been sealed since the postmarked date. This will stand up to a degree, oppisition can still whine about that though.
If you really want to make sure that your work is protected, you will want to obtain an SR(sound recording) This protects your actual recording so if someone did, say, use your recording in a commercial sans permission you would have grounds to press charges. This cost's $45, but if you plan on it getting decent circulation this is highly advisable.
http://www.copyright.gov/forms/
It's about halfway down the page, or just search for SR in your brower on that page.
If I remember anything I forgot I'll update
PS. I'm just a geek who know's WAAY too much about copyright law for any normal person, I think I remember hearing a couple here on UU were/are lawers. I'm guessing they will most likely know more than me
and by the way, congrats on the fan demand for CD!