Most sound decoders allow volume control of all the individual sound effects. It is not just one recording with all the sounds combined.
Tweaking the specific volume-control CVs, you can fine tune volumes of each sound effect (like horn, bell, engine sound, compressor, poppet valves, etc.), then you also have master volume control to set the overall volume. That allows for a fine adjustment of all the sounds.
I agree with previous posters: if the horn volume is too low, increase its volume, then decrease volumes of the other sound effects. Once you have the sound balanced to your likings, adjust the master volume to suit your taste.