To turn sounds on or off in your google maps please follow these steps. 

In your navigation screen click the sound Icon to choose the setting you want.

  1.  Muted: If there is a line through the speaker, it means the sound has been muted.
  2.  Alerts only: If there is a speaker with an exclamation mark, it means you will receive alerts only. 
  3.  Unmuted: If there is a speaker with sound waves, it means you are unmuted.

If your icon indicates unmuted but you are not hearing any sounds your phone volume may be set to mute.