You need to know which sound project is loaded into the decoder (I'm assuming these are equipped with the ESU LokSound Select Direct Micro boards). Unless you have been provided information on what sound project you have, you have to "sneak" into the info with Decoder Pro (and make sure you have at least the latest production release)
Go to the roster file for your loco (Program on Programming Track), and then navigate to the Information tab. Do a "read sheet"; on the right hand side, after a while, a project name will appear.
Now, go to the ESU Sound Projects page:
http://projects.esu.eu/ , and search in the LokSound Select Retail Sound files, or LokSound North American & Australian Factory Equipped Sound files sections for the name of the project (hint, do an initial search for GP38-2 to limit the choices). When you find the matching name, expand the entry and look in the "bulletin". It will give you a list of all the horns (and CV48 setting) included in that project.
It would sure simplify matters if Atlas would just include the bulletin with any of their sound equipped locos (or at least make it available on line)