sord. is one of the abbreviations that carries more than one meaning. Written out, it can be sordino or sordino — 2 separate directions that happen to shorten to the same letters. The score itself decides which: the surrounding passage, the instrument, and what the music is doing at that moment.
sord.
In a score, sord. can stand for sordino or sordino. Here is how to tell them apart.