Recording a message or music backwards onto a record that is meant to be played forward is called "backmasking". Musicians of all kinds have used this recording technique in the past for a variety of reasons. Some bands, particularly metal bands, have been accused of using backmasking to record demonic messages.
The hand gesture the Titans make while thrashing to metal music is often referred to as "the sign of the horns" or the "rock on" hand gesture. It's typically made with the thumb over the ring and middle finger. The Titans made this hand gesture with the thumb extended, which also happens to be sign language for, "I love you".
Raven is unhooded for the 72nd time.
This episode isn't the first time a Titan was disgusted by bone furniture. In "Booty Scooty", Cyborg in particular was creeped out by a piano made out of bone.
The demonic graphic for Demon's Tongue Mountain, first seen in "Rad Dudes with Bad Tudes", is reused as a stage prop at the heavy metal concert the Titans accidentally attend.
The amp that goes up to eleven is a joke from the 1984 film, This Is Spinal Tap, a mockumentery about a fake band called "Spinal Tap".
The style of metal and jazz fusion is most commonly associated with "jam bands" such as Phish and progressive rock bands like Pink Floyd and the Talking Heads.
Raven's metal hairstyle resembles Ashi from the fifth season of Samurai Jack.
Ironically, both Raven and Ashi are voiced by Tara Strong.
When the guitarist strums his electric guitar, the noise obviously breaks Cyborg's non-human eye but in the next scene, the eye is unbroken.
Robin asks the other Titans if they knew what a record was. None of them knew but however, Cyborg, in earlier episodes, seemed to know what they were.
Robin playing smooth jazz.
The Titans (excluding Robin) making stuff and saying, "SO METAL!"
The image gallery for Throne of Bones may be viewed here.