Learn French: Certains se chargent de la musique, n'est-ce pas ? - Some are taking charge of the music, right?

Here, Certains acts as an indefinite pronoun meaning "some people". It replaces a noun rather than describing one. Because it refers to a general group of people (or a mixed group of men and women), it takes the default masculine plural form, hence the -s. If it specifically referred to a group of only women, it would be Certaines.

Why do we use the reflexive pronoun se in se chargent?

The verb se charger de is a pronominal verb that means "to take charge of" or "to take care of" something. Without the se, the base verb charger just means "to load" or "to charge". You have to include the reflexive pronoun to get the meaning of taking on a responsibility.

Why do we say de la musique instead of just la musique?

The verb se charger always requires the preposition de to connect it to the thing being taken care of. Just like you say "to take care of" in English, in French you must say se charger de. Since "the music" is la musique, they combine to make de la musique.

Can n'est-ce pas be used for any tag question like "right?" or "don't they?"