Based on an explanation by Chacal on Train Sim.com, I opened the SD70 Horn.proxybin file, and made the required modifications. However, nothing changed. The horn sounds at the same volume in the F4 view as it does in the F2 view.
I added a .bak extension to the file SD70 Horn Sound.bin, which stopped all horn sounds. This leads me to believe the horn is aliased to another file.
Thanks in advance for any and all help - sometimes, doing file modifications is just plain frustrating.

Rob