On my first 2018 XLT, the low-note horn had already failed by the time I bought it, at only 7K miles.
On my current 2018 Sport, the exact same thing was evident, even though I'd bought this one used, at 79K miles.
In both cases, I had a warranty, but I just ordered the unit myself, and changed it out myself... as it wasn't worth it to me to give it to my crappy local Ford dealer shop for 2-3 days before they even got around to diagnosing the issue (much less ordering and installing the part).
Both of these replacement units only cost me around $45, including tax, which seemed very reasonable to me... and, again in both cases, no more issues with the horns since the change-out!