The most common reason that you may not be hearing sounds when using Tally Counter is that the Ring/Silent switch on the side of your phone has been switched on. The Ring/Silent switch is usually on the upper left corner of your device and if it is set to silent it will have some orange showing. Flipping the switch the other way should allow sounds to play again.
If your Ring/Silent switch is off and you are still not hearing sounds sometimes restarting your device can clear up the problem. Apple has instructions for restarting an iPhone or iPod here: http://support.apple.com/kb/HT1430.
Apple has a support document covering the functionality of the hardware switches on your device available here: https://support.apple.com/en-us/HT203017.