This is a Zelda Tears of the Kingdom side quest that is anything but complicated. However, you may have a hard time finding the location of this sinister chant in this new Quest for Truth.
A Sinister Song: Start the side quest
This quest is part of the 12 surveys that you must lead with Penn to be able to finish [En quête de vérité] of the Gazette des Bois and get the rewards. You will find it at Alpages Relayin the Firone region, south of the map. The nearest sanctuary is Utu Xoqi and the nearest point of interest is Lake Hylia.
Once there, you will find Penn either right in front of the Relay entrance, next to the fence, or leaning against a nearby tree. He will speak to you in a mysterious voice that scares the locals but can only be investigated at night. Use the campfire next door, with the pot, to use the “wait” option until nightfall and thus not need to pay for a bed at the Inn.
Where to find the mournful voice?
Talk to Penn again, who will tell you that the voice is coming from a little further away. Just follow him a little further north, into the Arafur Plain. There you will be let loose and will have to find where the sound is coming from. You have to identify a abandoned well, very exactly between the pond and the plain on the map (see image below).
Jump into the well and you will see that it is just a simple little girl humming because she likes the sound of her voice resonating. Take the opportunity to collect the few weapons left available, the flower bomb hanging on the wall or the Troglodus Lumos in the water.