Hello science fair student,
I thought I'd add my odd thoughts to Keith's.
I'm puzzled by your comment about not being able to record longer durations with Audacity. I just recorded about 90 secs. of sound, and I stopped only because it seems like it would run on and on. The entire record is on the display and can be examined.
If you still have trouble with Audacity, try http://www.avs4you.com/AVS-Audio-Editor.aspx
. Some features of the software require purchase, but I think the trial version does what you want.
If you have an old phonograph player or you're willing to buy a phono cartridge, you could rig up a way to hold the needle in contact with some part of the piano. By moving the needle around, you could record from the metal frame, the wooden sound boards, or any other part of the piano. Such phono pickups are very low output, so you might need a preamplifier. If you want to try this, let me know and I'll provide more info.
Good luck, WW