You will need to ensure you individual Timer Features are a percentage which you can hear.
Tap the Speaker icon (to the left of the Start Button) on the Timer setup screen and make sure the volume is turned up for individual Timer Features. 
More in “Timer ”
- Why am I being asked to download the bells and ambient sounds?
- I don't want the screen to automatically darken.
- Can I use the Timer offline?
- How do I manually add a Timer session?
- Why can't I hear the interval bells?
- I would like to change the background image for the Timer.
- Can I set a reminder to use the Timer?
- Can I add extra time if I go over my set duration?
- Why am I hearing notifications during my session.
- I don't want to see who I meditated with after my session.