Examine the speaker and decoder very carefully, checking that you didn’t pull a wire loose or that a bare wire is touching metal on the frame or shell.
If the speaker connections look ok, checdk continuity with a meter from end to end of both speaker wires.
Pop a speaker loose and nothing else, and loose sound, and odds are you disturbed the electrical connection.
Distortion, and often sympathetic resonant vibration, are often a product of an over driven amplifier. Turn the sound down till it is no longer distorted, and then a fuzz more, before drawing any conclusions regarding anciliary vibration.
I’m guessing that you broke a wire. Take the shell off, put the engine on the track and try it. You’ll probably need to take the speaker out of its enclosure to check the wiring.
You can check the wiring to the speaker with an ohm meter. Take the engine off the track, and touch the probes of the meter to the outputs of the decoder. It should be 8 ohms. (I think the decoder itself should be high impedance, right? So all the meter will really read is the speaker resistance. I’m not sure on that one.)