Service connection may not be established for disability due to impaired hearing unless the auditory threshold in any of the frequencies 500, 1000, 2000, 3000 or 4000 Hertz is 40 decibels or greater; or the auditory thresholds for at least three of the frequencies 500, 1000, 2000, 3000 or 4000 Hertz are 26 decibels or greater; or speech recognition scores using the Maryland CNC Test are less than 94 percent