Those were sold as a lone powered A unit with SignalSounds (a good sounding horn and bell) in 1997, with a B unit with full RailSounds following in 1998. There wasn't ever a B unit without sound, so either someone removed the sound system or you have a mis-matched pair.
The easiest way of solving the problem would be to find the proper B unit with RailSounds -- 6-18966.
TRW