Here is a list of the unlockable songs that can be used in certain stages. I believe this is most of them, but there may be more. Please add discovered ones to the list. Sorry if this is posted somewhere else.
Green Hill Zone Emerald Hill Zone Angel Island Zone Hydro City Zone Mushroom Hill Zone Death Egg Zone It Doesn't Matter Emerald Coast Green Forest Metal Harbor Special Stage - Bonus Challenge His World Kingdom Valley Endless Possibility Savannah Citadel - Day Sweet Mountain Toxic Caves Palmtree Panic Sonic Boom Stardust Speedway Super Sonic Racing Neo Green Hill Zone Leaf Forest Splash Hill Zone
HyperWaterVX, your aquatic defender! Rain is like an endless dream...
Green Hill Zone Original Marble Zone Star Light Zone Emerald Hill Zone Chemical Plant Zone Original Sonic 2 Special Stage Angel Island Zone Hydro City Zone Marble Garden Zone Mushroom Hill Zone Flying Battery Zone Death Egg Zone Open Your Heart It Doesn't Matter Emerald Coast Live and Learn Green Forest Metal Harbor Sonic Heroes What I'm Made Of... Sonic Heroes Special Stage - Bonus Challenge His World Wave Ocean Kingdom Valley Endless Possibility Windmill Isle - Day Savannah Citadel - Day Reach For The Stars Tropical Resort Sweet Mountain Toxic Caves PalmTree Panic Sonic Boom Stardust Speedway Zone US Diamond Dust Zone Super Sonic Racing Neo Green Hill Zone Leaf Storm Route 99 Right There, Ride On A New Venture High Flying Groove Un-gravitify Free Quick Trip To Paradise Race To Win I Am... All Of Me Knight Of The Wind Splash Hill Zone