You may try the following options:
- Go to Menu > Settings > Audio Settings > Voice Volume and manage the voice volume. You can also manage the audio feedback (hearing your stats during the run) there.
- Before you start running, make sure you've turned on the sound on your device.
- Use headphones for the workout.
- Use the Padlock icon in the bottom right corner to lock the screen instead of using only the lock button on your phone. Then the app will work properly, and you’ll be able to hear all voice instructions and follow your stats even on the locked screen.