Answered: VEXU speakers as part of custom sensor

Under the same logic that ruled Led lights as a legal component of a VexU sensor in part B) of this thread: VexU Bounds of Custom sensors,
would non-vex speakers be a legal component of a custom VexU sensor?

Yes, this is legal, provided that no other rules are violated in the process. R7d, part iii, would apply here, which states that any chosen audio must not be distracting and must be in good taste.

Also, please note that the the linked question referred specifically to using LEDs as status indicators for a custom sensor. VUR7 only provides allowance for custom electronics if they are used for sensing and/or processing. The speaker in question should be a functional component of a complete custom sensor unit, such as beeping to indicate that the sensor has detected something. Speakers that are included simply for the sake of playing sounds or music would not be considered sensors.