Yum yum! is used to say that food tastes or smells very good, and dig in! is another exclamation meaning: don't wait, start eating and eat as much as you like.
Vocabulary used in speech or writing organizes itself in seven parts of speech (eight, if you count interjections such as Oh! and Gosh! and Fuhgeddaboudit! ).
A lot of interjections are used to show strong emotions, whether positive or negative, for example, love, hate, surprise, boredom, anger and many more.
In this case, they were looking for an interjection that signals that a listener missed something, and prompts the speaker to repeat or rephrase the original statement.