Hi, thank you for reporting this. I performed a very thorough check of the current version of The Cave, including the latest "3.0.39" build, to make sure that we had not accidentally reduced or disabled the train sounds.
I checked all of the Lua code related to locomotives, cargo wagons, fluid wagons, artillery wagons, Train Interiors, the Main Locomotive, Train Shield upgrades, hidden Train Shield prototype variants, train creation and migration code, and the final prototype adjustments. I also searched specifically for every train-audio property that could affect the result, including "working_sound", "sound_minimum_speed", sound volume, sound scaling, audible distance, activity-based volume and custom train sound files.
I also cross-checked several different rolling-stock prototypes and hidden Train Shield variants to make sure that none of them were losing their original sound definitions or producing a data-stage error. I checked the archive itself for errors as well. The ZIP is valid, the relevant code loads correctly, and I could not find any train-audio-related error or incorrect prototype override.
The Cave does modify a few properties of rolling stock. For example, it changes the health system for Train Shield compatibility and sets Cargo Wagon inventory capacity to 80 slots. However, it does not modify the sound properties of the locomotive or any wagon. The hidden Train Shield variants are created by copying the original rolling-stock prototypes, so they inherit the original Factorio sounds without changing their volume or behaviour.
There is one line in the mod that disables a "working_sound", but I verified that it belongs only to an invisible accumulator used internally by the Train Battery power system. It is not a locomotive, wagon, engine or any visible train entity, so that line cannot reduce the actual train sound.
I also reviewed the complete mod changelog and found no previous entry where locomotive or wagon sounds were intentionally reduced, removed or replaced. Based on the current code and the available history, The Cave has never intentionally turned down the train volume.
There is one important detail concerning Train Interiors. When a player enters a locomotive or wagon interior, the player is teleported to a separate internal Factorio surface. The physical locomotive remains on the exterior surface, and Factorio does not carry positional sounds from one surface to another. Therefore, if you are referring to the sound while standing inside a Train Interior, the exterior locomotive’s engine and idling sounds may not be audible there. That is a consequence of the separate interior surface, not a reduced train-volume setting.
When you are outside and standing next to the locomotive, it should use the normal Factorio sound definitions and normal volume. If it is also unusually quiet while you are outside beside the train, please let me know whether this happens with the Main Train, an ordinary player-built train, or both. It would also help to know which Factorio version you are running and whether any other train or audio-related mods are enabled.
At this point, after checking the complete relevant code, several rolling-stock variants, the sound properties, the changelog and the package integrity, I cannot find any change or error in The Cave that lowers the normal exterior train sounds.
