Cant hear unless s8 is on speaker

[AC Question]

Is there a setting or anyone know why i cant hear callers unless my phone is on speaker

 

[anon(1499564)]

I have the same problem after 3 days and sent to service center..I was told is a defective motherboard and was replaced a new phone

 

[CKwik240]

Unlikely this is it, but I once left the Bluetooth on my motorcycle helmet on and didn't realize it. I couldn't hear any calls until I put it on speaker as it was going through the helmet. Just one possibility.