In this example, is the mbed connected to a special sound-producing device? What is it? http://mbed.org/projects/cookbook/wiki/MIDI

Most keyboards and Synths have an input/outout pair of connectors known as MIDI. (I have a Yamaha here with them on). They allow computers to communincate with them and record what you play and play it back amoungst many other things (note, it doesn't record the music like a tape recorder, it records the notes you play).

Try Googling MIDI and you'll discover there's nothing special about the sound devices.

Hmm, is there a cheaper solution than a Yamaha or a MIDI keyboard?

MIDI is Musical Instrument Digital Interface. They tend to be found on electronic musical instruments which are not renowned for their cheapness. If you don't have a MIDI capable instrument, what's your project?

Trying to make a cheap high quality instrument-esque... Might end up connecting the embed to existing speakers.

You're totally missing the point of MIDI. MIDI does NOT play music through speakers. It is merely a protocol that allows a computer (or Mbed) the ability to tell a musical instrument which notes to play. The instrument makes the sound not the computer (or MBed). It is not about recording and playing back music like MP3. I suggest you do some reading up on the subject.

Do you know how to send MIDI message from a musical keyboard to mbed via the USB pins? I compiled the "USBMIDI.h" but no response has been elicited from the mbed(program sets LED to flash when receiving MIDI message).