It seems that you have spent a considerable amount of time trying to regain the good sound after it is lost but have you spent anytime trying to lose the good sound. IOW have you done the mute/unmute or switching inputs etc repeatedly before you have lost the good sound to try and create the problem?
If you have some success on causing the problem on purpose it would give us another clue.
If you have some success on causing the problem on purpose it would give us another clue.