iPhone XR ear speaker has gone too quiet

If you mean on phone calls, try using vol up button during a call. It cannot be increased any other way.

Failing that, you should arrange a service or repair

https://support.apple.com/iphone/repair/service

Anything other than Screen or Battery may be a replacement phone and the price is in the article.

You can see additional service information within the following article: 

Get your iPhone, iPad, or iPod touch ready for service - Apple Support