Earphone volune

Do you have the volume limiter set? As SranvanKrA asked earphones all have different sensitivities and if you have low sensitive headphones and the volume limiter set, that could be the issue. In iOS 14 that is now called "reduce loud sounds" Very large over the ear headphones may also require a lot more power than an iPhone can produce.

Check -

Settings>Sounds & Haptics>Headphone Safety and see if "reduce loud sounds" is on

Are you using Apple AirPods? Then Try

1. Summon Siri, then say something like “Turn down the volume.”
2. Use either volume button on the side of iPhone.
3. Drag the volume slider in an app's playback controls.
4. Open Control Center, then drag the volume slider.
5. Drag the volume slider on the Lock Screen.
6. Turn the Digital Crown (AirPods Max).

Personally I use Higher end headphones like Shure, Etymotics and Bose. I don't mind the wires and all of them are plenty loud.

There is also this help file on the apple support site:

Go to Settings > Sounds (or Settings > Sounds & Haptics), and drag the Ringer and Alerts slider back and forth a few times. If you don't hear any sound, or if your speaker button on the Ringer and Alerts slider is dimmed, your speaker might need service. Contact Apple Support for iPhone, iPad, or iPod touch.

https://support.apple.com/en-us/HT203794#:~:text=Go%20to%20Settings%20%3E%20Sounds%20(or,%2C%20iPad%2C%20or%20iPod%20touch.

Which earphone is this?