message sound doesn't work

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The tone I've selected for incoming messages doesn't work. Well, almost. I can go to Settings>Notifications>messages, and select a tone. When selected, I can here the tone. Close settings, send myself a message from my PC and I get a sound and a note on my lock? screen. But, I get the tone once, sometimes twice. After that, I get no sound and not note on the lock screen. Back to Settings>Notifications>messages, and select a tone. Again, close settings, send myself a message from my PC and I get a sound and a note on my lock? screen. Again, once or twice. Sometimes, when I select a tone, it makes no sound. Have to exit Settings and try again - sometimes 3-4 times before I hear the selected tone. If I don't hear a tone on selecting it, message sounds don't work. FWIW, I have the same problem with reminders (Alarmed).

How do I get the message sounds to be permanent?