There is a tick box at the bottom of the preferences window to display or hide the volume control (speaker icon) in the menu bar. To completely silence alert sounds so they won't ruin your enjoyment of music for example, drag the slider all the way to the left and clear Play user interface sound effects. Some people don't like system sounds and they drag the Alert volume slider to the left to quieten them. Some people want to be alerted to events even if they are listening to music or watching a video, so they set a high alert volume so alerts can still be heard over the top of any other audio that may be playing. Notice that there are two volume sliders and the top one is for system alert sounds, and the bottom one is the audio volume for everything else. If you want to hear what each sound is like, just select it in the list after turning up the volume of course. These are system sounds and they are heard when certain events occur, like notifications, warnings, message boxes and so on. On the Sound Effects tab is a list of all the alert sounds. The Sound properties window is displayed. The Mac's audio settings are in System Preferences, so go to the Apple menu in the top left corner, select System Preferences and then click the Sound icon.
You might think that adjusting the audio volume on an Apple Mac is straightforward, but there are features you may not be aware of that can make life easier.