c'est sa mélancolie qui a trouvé un écho chez moi.

Bonjour Tuhin,

Using L'Imparfait would mean that it was a repeated action/habit that used to happen in the past. Using Le Passé Composé, as much as referring to a completed one-time action, also refers to the impact into Laëtitia's present: Barbara's melancholy impacted how Laëtitia sees her songs now.

Take a look here:

Imperfect vs. Passé Composé

I hope this is helpful.

Bonne journée !