You must beat special stage time attacks in under 4 minutes to unlock the Sound Test.

Unlockable	How to unlock
Hidden artwork 1	PGM 12, DA 11.
Hidden artwork 2	PGM 12, DA 2.
Hidden artwork 3	PGM 03, DA 0.
Hidden artwork 4	PGM 04, DA 2.
Hidden artwork 5	PGM 11, DA 0.
Secret special stage	PGM 07, DA 07.