The Musician is primarily a roleplaying role, however, this does not mean you cannot jam out in non-roleplay circumstances. You are a glorified Passenger (with cool sunglasses) whose purpose is to play midis on Instruments for the crew.
You are given a acoustic guitar, and a saxophone. You can use these to play music wherever you go. Your PDA can play music too! After selecting the "Music Instrument" option in the PDA's Menu, you can play Wonderwall without needing to use your hands to pluck some pesky guitar strings. Other crew members can join in with instruments of their own!