Cant listen to whatsapp voicenotes throught the earpiecer

I am using my iphone 8 plus using ios 11.2.6

The problem is that when I listen to whatsapp voicenotes using the earpiece, I cant listen snd a symbol comes on the screen showing that volume is turned off although it is turned on. The earpiece works perfectly in other apps like music and other stuff. What is the solution?

This is the symbol that comes up when i remove it near my ear