Here are a few things to try, in this order:
- Even though your device isn't in this state, follow the instructions on this page in the "If your screen is black or frozen" section to force restart the device: https://support.apple.com/en-us/HT201412 . Your device may be in some other bad state, and this may get it out of that state.
- Have your device set up in the manner you intend to play (e.g., if you play with earphones, hook them up now). Launch the game, then press the device's Volume Up button multiple times after the app is running. Confirm that there's still no sound even when turned all the way up in this manner. (iOS tracks several different volume levels depending on what app is in the foreground and whether stuff like headphones are hooked up, so just because the volume is fully turned up in another situation doesn't necessarily mean it's fully turned up in the situation you're in while playing.)
- Open the game's Options Menu and:
- If your game has volume sliders, turn up the volume sliders all the way. If they're already turned up all the way, turn them down and wait for a second, then turn them up all the way.
- If your game has button(s) for sounds/music, toggle them on. If they're already on, toggle them off and then back on.