Would you happen to have a photo showing how and where you mounted the decoder and speaker?
I am just embarking on modifying a series of the bachman 0n30 trams, where did you obtain the sound file, I haven't been able to find anything other than the digitrax generic heritage streetcar?
I am not an expert but I have never worried about which end of a resistor is the input end and I have never had any problems.
I don't see any reason why you shouldn't be able to reuse the resistor so long as the out put from the decoder doesn't exceed the voltage the LED would normal get from the track.
I am replacing the LEDs with bi-colour ones so I have a red at the rear and white at the front, the LEDs I have purchased come with a resistor, I will check the rating of the existing and new and advise if they differ.