Over at Train Sim Community, CentralJerseyRailfan has a EMD 567 sound enhancement pack that includes a large selection of horns. If I'm not mistaken, the DRGW SD40T-2's were delivered with Nathan M3's horns, although after the SP merger it wasn't unusual to have a Nathan P3 wind up on a DRGW tunnel motor.
The sound enhancement pack includes the M3 horn sound. As Buzz had mentioned, you'll need to do some work to adapt the SD40T-2's horn files to point to that M3 horn sound. It's doable with the help of TSTools. And if you have any DLC that includes F's or GP7's / GP9's, this sound enhancement pack applies upgraded prime mover sounds along with the correct horn for the appropriate railroad.
https://www.trainsimcommunity.com/mods/ ... -audio-fix