Also sometimes after leaving apps the audio is still playing through other apps.
Audio-centric apps should continue playing audio, when returning to the home screen. That is intended behavior. You shouldn’t get multiple apps playing though each other, though.
Force-quit the misbehaving app(s). Then try again.
If the issue persists, then force-quit the misbehaving app(s). Restart the device. Then try again.
If the issue persists, then try deleting the misbehaving app(s), and download those again from the App Store.
If the issue still persists after that, then contact the app developer(s).