iphone 8 earpiece speaker low sound volume even when fully turned up

Make sure that you are turning the volume up while you are on a call. Turning the volume up while not on a call only turns up the ringer volume.

If you still can't hear, you may have a hardware issue. Make an appointment at the Genius Bar at your local Apple Store for evaluation.

Genius Bar Reservation and Apple Support Options - Apple

Chrismufc79 wrote:

Do you know if there is a test they can do to check the earpiece speaker is functioning as loud as it should?

Well, when the speaker on my iPhone 6 went bad a couple of years ago, they were able to deal with it.