Answer:
The correct option is "En los conciertos de 2004, a veces no te gustaba la voz de la cantante".
Step-by-step explanation:
The sentence refers to an event in the past, which occurred in 2004. Therefore, the conjugation of the core verb of the sentence must necessarily be in the past tense.
Thus, since the core verb is "gustar" (like), and the conjugation possibilities are "gustaban", "gustas", "gusta" and "gustaba", bearing in mind that the verbs in the past end with the ending - ABA, the possible options are "gustaban" and "gustaba".
In this context, knowing that the sentence refers to a single singer, and not several, the verb must be applied in a singular and not plural, so the correct option will be "gustaba".