Mereka cukup mengulang bagian refrein beberapa kali sampai vokalis merasa puas.

In Mereka cukup mengulang bagian refrein beberapa kali…, cukup means “just / simply / only to the extent of”, with a nuance of “that was enough to achieve the goal.”

• cukup

  • Core meaning: enough, sufficiently.
  • In this context it can be felt as:
    
    • They just had to repeat…
    • It was enough for them to repeat…
  • Implies that no more effort was needed than what they did.

• hanya / saja

  • Both often mean “only / just” but without the nuance of sufficiency.
  • hanya is placed before the verb/noun:
    
    • Mereka hanya mengulang bagian refrein… = They only repeated the refrain part (and didn’t do other things).
  • saja usually comes after the verb/phrase:
    
    • Mereka mengulang bagian refrein saja… = They repeated the refrain part only.

So:

• cukup mengulang = repeating was sufficient to achieve the result (vocalist satisfied).
• hanya/saja mengulang = they only repeated (and nothing else), but doesn’t automatically suggest “that was enough”.