To get the Alto Card go to Tadpole Pond and visit Toadofsky. Here are the notes
Frogfucius Suite #18 So,La,Me,Re,Do,Re,Do,Re
To get the Tenor Card, come back to Tadpole Pond after rescuing the kids from Moleville. Here's the notes
Moleville Mountain Blues Mi,Do,So,Do,Re,La,Ti,Do
To get the Suprano Card, come back to Tadpole Pond after talking to the Star in Monstro Town. Here's the notes
Star Song La,Ti,Do,Re,So,Do,Re.Mi
With the Suprano Card you can now buy KeroKero Cola for 200 gold coins.