Perform the following test set-up to see if your sounds are working properly:
Set up a meditation for 10:00, set all other settings to 00:00 (preparation, interval, cool down)
Tap the meditate button to start your session.
As soon as the session starts, begin pressing the up volume on the left side of your device until you see it is all the way up.
Tap the reset button to end the meditation session early
Set up a new meditation session by setting all time settings to 00:00
Hit the meditation button again to start a session
When you hit the meditation button, you should hear a chime sounding.
Check the volume in your device settings area. Go to settings > sounds; Check the volume level shown below.
Other things to try:
Try a reset of the phone: (No data or settings are lost) You hold the sleep/wake button at the top of the phone, while holding down the home button. You have to hold these buttons together, and do not let go until you see a completely black screen with the white apple logo in the center.
Delete Meditate from your phone, and from iTunes, then on the phone go into the app store, and search for Meditate, and reinstall directly on the phone. (there will be no charge for downloading an app you already purchased).