First of all, bear in mind not all devices can send and/or receive sound through HDMI. A good test is trying to play music with another app. If sound also doesn't work, your TV might not be compatible with audio transport through HDMI.
In that case, you can try to plug external speakers to your iPhone / iPad headphones output.
Also make sure your cables are plugged all the way in and the volume of your TV is turned up.