This ep was big on tragic emotion with Juanito in continued peril, and the SdL sound department pulled two melodramatic/farewell leitmotifs from the soundbank that have been prominnntly used for Mafin before:
During the baptism of Juanito, starting over the anguished faces of the shows OG OTP, Andrés and Begoña, there are a little under 10 seconds of the melancholy piano tune that underlies Fina writing her farewell letter to Marta (ep. 378), and that also underlies Marta burning parts of her diary in late Season 2. The motif doesn't advance to the orchestrated variation here and shifts into another motif when the scene cuts to the phone.
Then there's the Melancholy Farewell motive known among the Mafin crowd as "Marta swaggers into the Warehouse After Hours instead of going to London and proceeds to call Fina Mi Mujer". Interestingly, it gets exposure - briefly, likewise only as the piano take, not advancing to the orchestrated version - while Marta muses what would be if Juanito didn't survive.
Those lines stood out (quoting from a conversation with @madronash here, and the from liquor cart meeting of the episode) for their callousness, and while it may have been a dramaturgical necessity to give these lines to just anyone so Andrés can reply with "I'd rather renounce Begoña than have her suffer through this", it may also be a commentary on Marta having suffered that deep kind of loss (not of a child, but of a partner, out of nowhere) and being marked by it.
Dark!Marta might be an obstacle for a reunion with Fina - who knows what they are foreshadowing? At least the musical layer does still allude to the pain, and not just to someone being more jaded by loss.
