I don’t think there’s any reason to think auditory stimuli are less relevant for infant development than other modalities. Clearly language acquisition is one of the major accomplishments in the first couple of years, and I suspect there is a lot going on a lot sooner than we can evaluate in the normal course of interaction with babies. Even at birth infants show an attentional preference for speech sounds over other sounds. At days to weeks, they prefer phonemes found in the language spoken around them to those found in other languages. And by 9 months their babbling utterances have narrowed to be primarily sounds found in the language they have been exposed to. As for deliberate instruction and reinforcement, that has been shown to have almost no measurable effect on language acquisition. While almost all cultures speak to children in a simplified form of their language, many make no effort to deliberately instruct children in sounds, words, and meaning, and these children acquire their native language at the same rate as those from cultures like ours.
As for the pathway by which a video might help language acquisition, it is essentially the same as that by which watching and hearing adults in the environment does. Passive exposure to language and self-reinforced practice seems sufficient almost by itself to ensure normal language acquisiton. However, the contribution of a video seems, I agree, unlikley to have any meaningful impact either way. I can’t view the entire article since it is limited to subscribers to the print version of the journal, and I’m no sociologist so I’m not familiar with the evaluation tools (MacArthur-Bates Communicative Development Inventory (CDI)), but I know that much of the data was gathered by telephone survey, and though social scientists will hate me for it, I’m not a big believer in the validity of such tools for behavioral research. Hard to assess the accuracy of self-reporting, lots of effects of how questions are asked, etc. But I guess I can’t say for certain that the results aren’t valid. Still, given the complexity of early childhood development, I wouldn’t be either rushing out to buy the videos or to burn them. I watched several of them with my daughter when she was smaller, and they were entertaining for short periods, but I’m not too worried about how they’ll effect her college admissions chances