Rashi (on 12:30) quotes two explanations from the Midrash. One, if there was no firstborn males, then the oldest male was considered a firstborn male. Two, the firstborn could refer to either the firstborn to the father or to the mother, and since there was a great deal of promiscuity in Egypt, there could be many firstborn males.
Hizkuni (on 12:31) quotes two other explanations from the Midrash. One, from R. Natan that the Egyptians used to make a memorial to their dead, and during the plague the memorials were destroyed, which was equivalent to having the person killed. Two, the Egyptians used to bury their dead by their homes, and during the plague the dogs dug up the graves. This act was considered by the Egyptians as being equivalent to having the person killed.
Amos Chacham (1991, p. 202) suggests that the Egyptians used to live together as clans, and then the verse means that there was no clan that did not have at least one firstborn male who died. Yet, even if one accepts that house means clan, then still the clans would have to have been quite large that every clan would have a firstborn son.
A different possibility is that reference to the dead in every Egyptian household in 12:30 includes the animals that died in the plague. The previous verse, 12:29, records that the plague "killed all the firstborn in Egypt, from the firstborn who was to inherit the throne to the firstborn in prison and all the firstborn animals." Also, when Moshe warned Pharaoh about the tenth plague, he mentioned the death of the firstborn animals, 11:5, and the death of the animals is the source for the later law that a person has to redeem his firstborn donkey, 13:14,15. We see that that the death of the animals was an intrinsic element of the tenth plague and maybe then the idea that there was no Egyptian home without a death also includes the animals that died in the plague. 12:30 does not specify that the reference is only to humans, and people cry about the death of their animals. Note, I doubt that the firstborn animals died in order that the Torah could state that in each house there was a death, but once the firstborn animals died, then this led to the result that there was a death in each Egyptian house during the tenth plague.
Bibliography:
Alter, Robert, 2004, The five books of Moses: A translation and commentary, New York: W. W. Norton and Company
Chacham, Amos, 1991, Da'at Mikra: Commentary on Shemot, Jerusalem: Mossad Harav Kook.