For owned trailers or unsupported third-party mods, the developer included a manual workaround via the menu: Navigate to Audio Settings .

: Functional, though you may need to disconnect and reconnect the trailer once to trigger the sound cycle. The "Voice Navigation" Workaround

Select from the voice navigation dropdown. Click Preview (this activates the sound in the background).

: Adds a constant engine hum that is audible both outside the trailer and from within the truck cabin.