I think I witnessed a folklore legend.

So first of all, I am not a big folklore believer. I think it is interesting and fun to look into, but I have never believed in any folklore/myths.
However, last summer, I was taking a hike in Brittany. I found a spot that was essentially many cliffs and rocks, right above the sea, with the waves crashing below. There were also a lot of caves on this cliff-face. I climbed down with two family members. We had done this several times before. There was nobody around.
The tide was coming in, in fact we were a bit scared, so we decided to start climbing back up.
However, on the way, I hear a noise coming from a cave.
The cave is very deep, so much so that you could only see the entrance and then it was just pitch black. The entrance was fairly small so I couldn't see much, and there was some sea-water (from the tide) already starting to close in on the cave, so I didn't want to go in.
But I heard this noise. It sounded like singing. More like chanting. In fact, the closest way I'd describe would be as Gregorian monk chanting (you can type this in on Youtube and you'll find something that was very similar to what I heard). Naturally, I thought that it must be some sort of choir in the cave. My family members were hearing it too. One of them shouted into the cave, saying 'who are you', but the singing sounded completely unphased, as if they hadn't heard us. I must mention that it sounded like there were minimum 5 different voices signing, in perfect unison. It was beautiful to hear.
It was quite hard to hear the chanting because they were so far in the cave, in fact at times, with the waves crashing, it was quite faint, but it was definitely there. Eventually, we decided to climb back up to the top (due to the rising tide).
When we got back up, we decided to do some research. We guessed it must be a group of choir singers, probably taking part in some tradition. We asked the people who owned the cafe right next to it if they knew of any monks, they said no. I began to google, to see if I could find anything, I could not find anything at all. I even went into the local church and asked if the priest knew of anything to do with this, he had no idea. What I find strange is that these singers were so far into the cave, with a rising tide that would have killed them if they had stayed 5 minutes longer, without any acknowledgment of us shouting 'who are you'. It made me think that perhaps they were not humans, but something else.
I have done some research. I thought potentially they could be korrigans. I'm not sure. I still have no idea what those were. Any ideas?