First, be aware that not all markers on the map will play audio stories. Just Ahead is intelligent. It knows which way you are traveling, and will not play a story if you are driving the opposite direction. It also will not play a story you have already heard.
If you seem to have a problem, try closing the Just Ahead guide, then reopening it, using the audio setup utility that appears each time you open a guide.
There are several other reasons why you may not be hearing your audio:
Phone volume. Your volume might be turned down. Adjust the volume control on your phone to make sure it is set appropriately. If your phone is connected to another audio device, i.e., a car stereo, bluetooth speaker, headphones, etc., turn the volume up on that device.
Source volume. Your phone might be connected to a headphone jack or bluetooth, and that device might have the volume turned down. Play some music in another app on your phone to see if you can hear that music.