Open a third-party app like Instagram and browse through stories.
Also, turn up the Volume using the side buttons.
Make sure that you disable any Focus modes configured on your iPhone.
Is the Silent Mode in iOS 15 muting audio playback in third-party apps? Are Instagram stories silenced when the iPhone Silent switch is ON? Are we dealing with a No Sound bug or is this how Apple wants Silent mode to impact apps, system-wide in iOS 15? How To Test iOS 15 Silent Mode Issue