I'm going to assume you're attempting to merge the music into one track (if you aren't, my apologies). I also haven't used Audacity before, so I'm unsure what its capable of (though I should look into it at some point).
Anyways, what I can think of though is to convert the selected tracks into .wav's and then use Sound Recorder (comes with Windows) to cut the songs to the desired portions (then if you want, convert them back to the original file format). I realize this is probably a very inconvenient way to get about this, but it's the best I can think of for the moment.
As for recording your voice, Sound Recorder can also take care of this. Make sure your microphone works, hit the record button, and start talking. This will be recorded as a .wav file.
Note that you'll need another program to convert file formats. I use dBpowerAMP Music Converter to convert files.
Sorry again for such an inconvenient method, but it's how I get about things (if it helps any) with a simple Windows program. Good luck on your project.
P.S: Taking a quick look at Audacity, something tells me you're better off there. But if that doesn't take off, you could try what I just mentioned.