Successfully complete the indicated task to unlock the corresponding instrument at the "My Closet" menu in the Rock Shop:

Bomb bass: Successfully complete the Impossible Bass Challenge.
Bone microphone: Successfully complete the Impossible Marathon 2 Challenge on the microphone.
Clear Fender American Deluxe Stratocaster HSS: Join the Rolling Stone Rock Immortals list.
Clear microphone: Successfully complete the Rock Immortal Setlist in World Tour mode on the microphone.
Glowing drum set: Join the Rolling Stone Rock Immortals list.
Goat head guitar: Successfully complete the Impossible Marathon 2 Challenge on guitar.
Gold bass: Successfully complete the Impossible Bass Challenge.
Gold drum set: Successfully complete the Impossible Drum Challenge.
Gold Fender American Deluxe Stratocaster HSS: Successfully complete the Impossible Guitar Challenge.
Gold microphone: Successfully complete the Impossible Vocal Challenge.
Silver bass: Successfully complete the Final Band Challenge on bass.
Silver drum set: Successfully complete the Final Band Challenge on drums.
Silver Fender American Deluxe Stratocaster HSS: Successfully complete the Final Band Challenge on guitar.
Silver microphone: Successfully complete the Final Band Challenge on the microphone. Bonus instruments
Successfully complete the indicated task to unlock the corresponding instrument at the "My Closet" menu in the Rock Shop:

Bomb bass: Successfully complete the Impossible Bass Challenge.
Bone microphone: Successfully complete the Impossible Marathon 2 Challenge on the microphone.
Clear Fender American Deluxe Stratocaster HSS: Join the Rolling Stone Rock Immortals list.
Clear microphone: Successfully complete the Rock Immortal Setlist in World Tour mode on the microphone.
Glowing drum set: Join the Rolling Stone Rock Immortals list.
Goat head guitar: Successfully complete the Impossible Marathon 2 Challenge on guitar.
Gold bass: Successfully complete the Impossible Bass Challenge.
Gold drum set: Successfully complete the Impossible Drum Challenge.
Gold Fender American Deluxe Stratocaster HSS: Successfully complete the Impossible Guitar Challenge.
Gold microphone: Successfully complete the Impossible Vocal Challenge.
Silver bass: Successfully complete the Final Band Challenge on bass.
Silver drum set: Successfully complete the Final Band Challenge on drums.
Silver Fender American Deluxe Stratocaster HSS: Successfully complete the Final Band Challenge on guitar.
Silver microphone: Successfully complete the Final Band Challenge on the microphone.