I missed this on my first playthrough so here's a guide to save you some time.
This occurs a couple minutes after you unlock the fifth language from the Mother "snow dog with armadillo skin and a rhino horn" thing. After you bring her four children to her, she gives you the fifth song and you hover across a gap. This leads you into a room where you ascend by riding the back of one of the baby snow dogs. The top of that room leads to the video below.
Once you're in that area, just use your map to find the other Fes as they are on the tops of trees. Climb up the trees, grab them, and when you land on the ground, sing to the Fe until the bubbles come out of your mouths and touch, signaling you've communicated. The Fe will then follow you. Do this to all 4 and the achievement's unlocked.