A few chapters ago, Akira stated that his foresight had started to fail since Lucas' flowering. And now, the Pope is also wondering whether this has to do with the tower's predictions failing. What only we readers know/remember is that the voice is what caused Lucas' flowering, because of the "vision" it led him to. Whoever sent him that vision did so to trigger the flowering.
Although it was said that Lucas can "see sounds", the only sounds he's been seeing were the ones he himself produced. Why would these be any different ? His sound constellations are able to directly communicate not only motor intents, but also feelings and desires. It would not be a stretch for them to be able to communicate memories as well.
Moreover, the matter of multiple timelines competing for actualization is growing ever more prominent, with at least two characters able to interfere directly with the flow of time, and the whole setting being the result of interferences from the future.
I'm calling it now: the source of that "voice" was Lucas from another timeline.