Low volume in iphone 8 plus earpiece

The earpiece might be dirty.

Try to gently clean it using a strong flashlight, a toothpick, a soft brush... and a lot patience.

If the issue persists, check also Settings>Accessibility>Audio/Visual, then adjust the Balance slider. Set the audio volume balance to the center.

If the issue still persists, make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.