Since different species have different audible wavelengths, I bet the Universal Translator has the ability to pick up wavelengths well outside the human range.
Engineers switching their UTs into diagnostic mode to “talk” to their machines. If it’s making any weird grinding/vibrating sounds they need to know, even if it’s outside their audible range.
Also, augments reversing that mode to turn their UTs into noise canceling headphones. Ferengi using that mode to help fall asleep. Or deliberately not including that mode in their UTs to adhere to some guideline in the Rules of Acquisition (something something always keep your ears open).











