Here's me doing Sarani Tatakau Monotachi (the FF7 boss theme) with only my voice.

saranitatakaumonotachi.wmv - YouTube

I used Audacity to record multiple separate channels, which is actually really easy. This took me like fifteen minutes to record a couple of months ago, and just now I had the courage to stick in in Windows Movie Maker so I could share it via YouTube (granted as an unlisted video but still.) I did this without listening to the original tune as I went along, and did the guitar part first, a costly mistake as it kept making me get out of breath and go out of time, but my friends all insisted I not do an improved version so...

Come on, grab a mic, open Audacity or your favourite audio editor and try something like this yourself