Specifically, it plays a 15-second version of "Farewell" from Gradius II GOFER no Yabou. We found an actual recording of the music box in action here. This item was only available as a mail-in offer to people who purchased three Gradius soundtrack CDs.
We don't even get the opportunity to buy soundtrack CDs most of the time over here, and Japanese fans get to listen to those soundtracks as music boxes. Now we just have to ask ourselves: is the best fifteen seconds of Gradius music ever in a box worth $99?