I had to search a little to get a screenshot of the maps once directions have started.

When looking at the maps for directions, once you have started, you should see that speaker icon on the right side. If you do not see it, just tap on the map in that area and it should appear. There will be 3 choices. You will see a speaker icon, then a speaker icon with the small triangle beside it, and a speaker icon with the line through it. The speaker icon if tapped on will provide all sounds. The one with the triangle beside it will just provide important information, and the one with the line through it will mute directions.