Hello Tara_chapagai,
Thank you for using Apple Support Communities! I understand that you're experiencing issues with sound playing at a lower than expected volume on your iPhone X. While we're unable to contact your nephew directly as requested, I can help provide some additional troubleshooting steps you can reply to him.
I would like to start by mentioning that the speaker is located on the bottom-right of your iPhone near the Lightning connector. If this is blocked by a case or other material, it could cause the lower volume in portrait mode compared to when your iPhone is in landscape mode. When you use apps such as TikTok and the sound is low, what happens when you try to increase the volume from the Control Center or using your Volume up button?
Adjust the volume on iPhone - Apple Support
The following support article also provides additional troubleshooting steps to take for this issue:
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support
Take care.