How do I unlock my iTunes account when my old e-mail is unavailable?
Some social engineering solutions :


Your iTunes account was presumably associated with your credit card, yes?  You could attempt to prove you identity to Apple via that credit card connection.  You could warn them that you plan on contesting credit card charges if they cannot restore the account. 
If Apple won't help, then collect your old credit card bills with iTunes charges, and speak with your credit card company about contesting those charges.  You're beyond the 60 day period, but maybe not the 1 year "claims and defenses" period.  I doubt you'll get your money back, but you have a legitimate dispute with Apple, so stick them the dispute resolution merchant fees, which might cost as much as $30 per song.  Do not lie to your credit card company, simply push the dispute process as far as possible, the further it goes the more money they charge Apple. 
You could theoretically sue Apple in small claims court.  Imho, this sounds like way way too much hassle for a few hundred bucks.  But maybe you could get your account back by contacting Apple's legal department in writing. 


Two technical solutions :


You say you still have the files themselves, yes?  There are a variety of tools for removing iTunes DRM.  A priori, I'd imagine most/all require authorization since that's the easiest way, but perhaps some clever one does not.  Try any that look promising.
Do you have an iPod?  If so, try Graeme Hutchison suggestion.  If it only give you encrypted music, then theoretically the iPod knows the authorization keys, meaning you've a slim chance someone discovered a trick for using the iPod's authorization to decrypt your music.  Google isn't finding anything for me though.


As an aside, you should never buy DRM protected content in the first palce because : (a) Eventually you'll lose access one way or another, often by the company discontinuing the service. (b) Your supporting the DRM technologies which create these problems.  Any books, movies, or music distributed with DRM are also available without DRM if you look.