1) Your device "settings">"sounds"
2) Both volume buttons on the side of your device
If you still get no sound, please restart your iPad.
Please note that the old Tabs HD app is no longer updated so it may have bugs. We released a new cross-platform app Ultimate Guitar: Chords&Tabs to which users of the Tabs HD app have the lifetime premium access. So simply download it and log in to your UG account to enjoy the brand new app!