A computer can have many audio outputs at once: internal speakers, a headphone socket, a monitor over its video cable, a docking station and any Bluetooth headset that has ever paired.
The system picks one default, and it changes that default automatically when devices appear. Connecting a monitor with speakers can silently move your audio there.
Volume exists at several levels too — the application, the system mixer, and sometimes the hardware itself. Any one of them at zero produces total silence.
Only after those are ruled out is a driver or hardware fault worth considering, and those usually cause distortion or crackling rather than clean silence.