This is the most interesting crossing of all: the same Polish surname can come from two different places, and one of them is this family's.
On one hand, żmuda in Polish means tedious, unrewarding work; from the old verb żmudzić, to waste time. A nickname, like so many.
On the other, Żmudź is the Polish name for Samogitia —the same region Zmud comes from—, and from it come Żmudziński and Żmudny, which marked someone who came from there.
In other words, there may be Polish Żmudas who share this family’s etymology without sharing its lineage, and others who do not even share the etymology. Two surnames coming from the same word does not make them relatives: it means two people, in two centuries and two countries, were identified by the same place of origin.
And here is the point that matters most on this whole page: in Argentina, a Polish Żmuda or Zmuda became Smud by the same route as a Baltic Zhmud. Customs did not distinguish etymologies. There are people named Smud in this country who are not related to this family and whose surname comes from here, not from Ludza. Sharing the surname, the country and even the century of arrival is not enough: a document is needed.