A. Check your iPhone's volume settings. Go to Settings > Sounds & Haptics and ensure that the volume slider is set to an appropriate level. Also, check if 'Change with Buttons' is enabled so you can adjust the volume using the physical buttons on the side of your iPhone.
B. Try restarting the app. Double-tap the Home button (or swipe up from the bottom of the screen on newer models) to view your open apps, then swipe up on Soul-Chat to close it. Reopen the app and check if the volume issue persists.