Final answer:
To keep audio playing on an iPhone when locked, use the 'Guided Access' feature in Settings > Accessibility. Enable Guided Access, open the app you want to play audio from, triple-click the home or side button, and tap 'Start.' Lock your iPhone, and the audio will continue playing without interruption.
Step-by-step explanation:
To keep audio playing on an iPhone when locked, you can use the built-in feature called 'Guided Access.' First, go to Settings, then Accessibility, and enable Guided Access. Next, open the app you want to play audio from, triple-click the home button (or the side button for iPhones without a home button), and tap 'Start' in the top right corner of the screen.
This will lock your iPhone into the app and prevent you from accidentally exiting it. Now, you can lock your iPhone, and the audio will continue playing without interruption. To exit Guided Access, triple-click the home button (or side button) again and enter your passcode.