Audio is breaking up when screen goes off, both on and off speakerphone

It sounds similar to what it sounds like when a bluetooth speaker/headseat is too far away from the phone, like garbled, rapid drops in the audio. Moving the phone slightly away from the head (to activate the backlight), or turning it on with the on-off switch when on speakerphone, fixes the problem. But as soon as the light is off again, the problem tends to return.

I think the problem does not happen when on a HD call.

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