Earlier today, I pushed the 1.0 release of NXT Band to GitHub. NXT Band allows OS X users to record and play back LEGO NXT melody files. With NXT Band's on-screen piano keyboard, you can quickly plunk out tunes and save them in the RMD file format. MIDI keyboards can also be used as input sources, and notes can even be drawn by hand in the familiar piano roll view.
NXT Band can be obtained from its Downloads page on GitHub. Feel free to play with the source code and report any issues you encounter!