Final answer:
The two kinds of underwater music systems, which are not specified, allow synchronized swimmers to hear music and perform their routines underwater without hindrance. One kind of system may involve speakers in the pool, while the other kind may utilize bone conduction technology.
Step-by-step explanation:
The two kinds of underwater music systems that are mentioned in the question are not specified. However, based on the given context about synchronized swimmers wearing earplugs to protect themselves against water pressure, we can assume that one kind of underwater music system is played through speakers in the pool, allowing the swimmers to hear the music while underwater. The other kind of underwater music system could be using bone conduction technology, where the music is transmitted through the swimmers' skulls and directly to their inner ear. Both systems enable the swimmers to hear the music and perform their routines underwater without hindrance.