Hello Ravart,
Thank you for using Apple Support Communities. Based on your post, your iPhone XS Max volume isn't the same. We'll do our best to assist.
You mentioned this happens through Bluetooth, as well. This step will help with the physical speaker though: Cleaning your iPhone
However, this might be more related to what's going on: If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
Does this only happen for specific types of audio, like calls or only music?
Does it help to disable Wi-Fi from Settings > Wi-Fi? Then disabling VoLTE in Settings > Cellular / Mobile Data > Voice & Data?
What happens when you create a voice memo using the Voice Memo app? Are you able to play it back on another device to test if the issue is only happening on your device?
Hoping this helps out, take care.