John is giving you good advice. If you do NOT use the leading AUX1 key, when you press the 1 or 4 keys you will hear a single bell ring that gets softer or louder with each key-press. When you hear the ring on each press, that is your audible cue that you are adjusting the overall, or "master" volume.
If you DO press AUX1 before the 1 or 4 key, you'll hear the background sounds get louder or software, but you will NOT hear the bell ring. In this case, the lack of the bell ring is your cue that you are adjusting only the background sounds, or the "blend" volume. In this case, the level of speech, horn and bell and a handful of other sounds will continue to play at full volume, or reduced volume depending on the current master volume setting or the setting of the physical volume knob on the locomotive.
Give it another try, I think you'll get it!