Realizing that most of the midi's are midi versions of popular songs, I'm wondering: what are the names of the songs?

Just use the numbers of the songs in the level editor for the "Original-Dirt" style (at least that's the one I'm using)

For example, I know #7 is "Istanbul (Not Constantinople)" by They Might Be Giants", and #6 is "Birdhouse in Your Soul", also by TMBG.

Anyone know any others?

Ah, you mean the Cheapo music.

Being pop songs, this means I probably won't recognize any of them off top of my head.  ;) But, it's not too difficult to find a MIDI-editing program and open up each MIDI to find out the info.  Most MIDIs have those metadata such as the name of the composition and who sequenced it embedded in the file.  Alternatively, if you can't find a suitable MIDI editor, a hex editor should work pretty well too, since embedded text data are stored as plain text inside a MIDI.

You can extract the MIDIs from the styles as follows:  go to the level editor and playtest a MIDI.  While it is playing, Alt-Tab to switch to something else, then run Windows explorer or whatever you use, and go to the folder where the Cheapo level editor is located.  You should find either a TempMidi or MidiTemp file (forgot which one is for the editor and which one for the game).  Just make a copy of that file and voila, you have effectively "extract" the MIDI that is playing.

Heck, you might even be able to just open up default.sty in a hex editor and find the names of some to all of the songs.  This depends on whether MIDIs are stored compressed or uncompressed inside a style file.

Only one I know is #14, "Don't Speak" by No Doubt. (Great song.)

#12 and #19 are by the Spice Girls, but I don't listen to that sort of **** so I wouldn't know the titles.  :P

Now that I think of it, you can probably just play the MIDI in Windows Media Player or the like, and it should show you the relevant info.