I had this exact problem yesterday, no sound while in navigation. It has to do with the volume of the voice, however I have not found how to set it normally. what I did was this,
Ok google now - nearest starbucks - it opens the google window and the voice is suppose to tell you nearest starbucks. Listen for it and tap the down volume, then press volume up to raise volume. Basically mine was max volume but so low I couldnot hear it, pressing the volume down seemed to "reset" the volume and I was able to put the volume back all the way up.
Problem is you have to do while the voice is talking to you or you get the phone ringer volume.