Providing a model number may help with a more definitive answer. I can tell you that I also have a pre-Bluetooth Lion Chief locomotive and according to its manual there is no control for the volume, only the on/off switch.
On later models with Bluetooth, it is possible to control the volume either from the remote or the app.
The only ways I'm aware to lower the volume on the early models is either by adding restive circuitry between the sound board and speaker or by some physical means of muffling the sound coming from the speaker.
