The "Ummmm" we see in the present example, expresses doubt in what was just said (written, really), and foreshadows that the statement to follow (from speaker #2) may be disagreeable to the other person (speaker #1).'

Similarly, it is not uncommon to find the word "Hmmm" as a preface to some statement. This expresses a notion of "I will need to think about this." But in some contexts, "Hmmm" can convey skepticism. In real-life conversation, people frequently exploit this ambiguity, as a clever way to signal that they are skeptical but will not state so out loud.

In this case, it signifies skepticism.