Volume too low at highest setting

Have you got any blockages of any kind in or around the speakers? Does your case block the speakers?

This article is designed for calls but still gives some handy advice.

If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support

More and more users are reporting this problem on different iPhone models. I believe I have tried all of the suggestions that have been made in the responses to these low volume reports. I have to use the speaker to hear most phone conversations. I have reported this issue on the Apple feedback page. It would be helpful if you reported the problem there as well.

iPhone 8 v 13.5.1