Thank you for the detailed answer, I really appreciate that. And you're right, I should have mentioned the context - that was a description for different songs of an indie band, so probably that makes sense. Anyway, I think I get it now:)

There are a few possible answers to this, depending who used the expression and in what context. 'Solid' to describe something (like a song) is a pretty common slang of saying something is really cool or excellent. 'Liquid' can mean the almost the exact same thing, describing something as amazing or excellent. 'Liquid' is slang that is really uncommon in Canada and the U.S. though. Another meaning when someone describes a song or music genre as 'liquid' - ie. liquid funk, means the music flows really easily, is of a more easy listening style. Saying the music feels like the easy flow of liquid, like water flowing in a creek or a river. Very peaceful and non-aggressive styles of music and song. Hope this helps!