Check that you have not accidentally muted the laptop's volume.
Otherwise, open the "Start Menu" and navigate to "Control Panel" > "System and Maintenance" > "System" > "Device Manager." Locate your audio device and ensure that there are no device conflicts, i.e., red Xs or yellow exclamation marks. If there are errors, try rolling back the audio driver or removing the current driver and installing a new one. You can find updated Realtek drivers for this laptop on Acer's support website: