Phone only vibrates when sound is on

[Android Central Question]

Hello. My phone only vibrates when receiving a text or a call, but the sound is on and notifications are on. The phone works like it is on vibration mode, but it isn't. If I connect my headphones I can hear notifications. During the first part of the day, I keep my phone on vibration mode and when I switch it to sound mode, the phone works like it's still on vibration mode, it only vibrates when getting a text, a call or any notification. I tried to restart my phone and after that the sound works but only for a few days, then I have to restart it again if I want the sound to work. I have a Sony Xperia XZ1 with Android 9. Does anyone know how to fix this problem?