The music is kinda important element. The sirens sing to hypnotize their pray, their voices is all about driving humans crazy. You can see in in a few scenes the magical power of their singing and how thy affect humans, that consume their voices like a drug. So I think it was very much intentional and justified.

And it is a different kind of storytelling, it is not direct, it doesn't show and explain motivations, there are no direct conversations about feelings and characters hide their emotions. Everything is suggestive and almost clear intentionally, it's a very classical approach in European cinema to storytelling. We are so used to obvious Hollywood style stories, where everything is just explained, where all the reasoning is very clear, that movies like this are deemed strange and lacking something. I loved it, this restrain, these half spoken sentences, these unexpressed feelings. This feeling of lacking is what makes it so good and memorable.

In perhaps a metaphorical approach, I found this lacking kinda fascinating and imperative. The entire premise itself shouldn't make sense. (I honestly couldn't figure out how or why the mermaids went from that opening scene, luring the two band mates, to being contracted by the band.) The visuals of the movie and the music, of which I, personally, thought was catchy, are supposed to lure the audience into this world that shouldn't be. The story involves these myths morphing their image and magical status in order to continue living in the human realm, the realistic realm. I think the film is then an effort to keep some of that magic, by giving us a type of story that looks and sounds nice, but very deceptive, like sirens luring in the sea. I don't know, this is just an inflated opinion of mine. I can see why you wouldn't enjoy it as much.

Way late to the discussion but I recently saw the flick and wanted to chime in a few points I learned from the commentary.

1. (should be kinda obvious but) this story is set in the 80s with 80s pop/dance tunes. Not just that but Polish 80s pop/dance tunes. Tight genre for sure, so I can understand some people not liking the songs. But they were perfect for the vibe. Kinda like Eddie & the Cruisers, you better like the musical genre or you'll be wasting 90 mins.
2. The filmmakers said this is supposedly Poland's first musical. Like, ever. So rather than copy any particular style (Gene Kelley, Mamma Mia, etc) they decided to include styles from across the entire spectrum. So you'll have some numbers that are lavish MGM type dance extravaganzas followed by intimate diegetic songs. It can be disorienting, but once you realize that's the intent, it's really interesting.
3. This is a dark comedy. I think that's why certain bizarre plot leaps happen, such as 2 mermaids being found in the river and suddenly jumping ahead to them singing at a dance club. I'm a fan of surreal dark comedy (Being John Malkovich, etc) so I think this quirky style worked brilliantly.
4. If you haven't seen the bonus features, there's a FANTASTIC song that got cut and I have no idea why. It was supposed to be their 1st song when they are auditioning for the club owner and it's my favourite part of the movie. I really wish they had kept it because it's a great song, wonderful scene, and it might've eased people into the fantasy a little better. Here it is on youtube: The Lure - Drunk, Lonely