In the Sophont creation rules, Part 10 (page 554), when generating the Elements for Hearing sense.
It is entirely possible to generate hearing frequency and span (the span of frequencies the sophont can hear) and voice and range (the range of frequencies the sophont can vocalize) which don't overlap. Meaning a sophont can speak (or otherwise vocalize) in a frequency they can not hear.
The obvious solution is to generate both frequency and voice with the same flux roll, meaning the vocalizations are always centered on the same frequency as the hearing range. Which makes a sense the way vision is centered on the star central wavelength.
It is entirely possible to generate hearing frequency and span (the span of frequencies the sophont can hear) and voice and range (the range of frequencies the sophont can vocalize) which don't overlap. Meaning a sophont can speak (or otherwise vocalize) in a frequency they can not hear.
The obvious solution is to generate both frequency and voice with the same flux roll, meaning the vocalizations are always centered on the same frequency as the hearing range. Which makes a sense the way vision is centered on the star central wavelength.