One thing to keep in mind is variation in any form. Dogs — like humans — prefer it if you mix up the playlist, something Evans said was exhibited by the dogs in his study, who eventually got bored of hearing the same tunes repeatedly for a week.
However, not much has been studied regarding music that incorporates singing or speaking. Evans is curious to study how dogs react to music and voice together, a thought that runs parallel to anyone who has left the television on for a dog. "Music and speaking are difficult," Evans said. However, if the television's on, "it gives them the same kind of [feeling] of people doing stuff and normal household noises . . . It's a sort of substitute."