Rock Band Fender Mustang Pro-Guitar Rock Band 3 - Manual (Page 13)

FAQ
1. Can I use the USB dongle that I use to play Rock Band to connect to a PC with MIDI
functionality?  No, to use the MIDI functionality of the guitar, you must connect to
another device using the MIDI Out port on the guitar.
2. If the guitar is considered a ‘real instrument’ why doesn’t it make any sound?  This
is true for many MIDI Controllers. One of the key features of a MIDI controller is its
ability to generate sounds through other devices such as: drum machines, organs and
sound modules. One controller can be used to drive all of those devices and be small
enough to carry.