Answer:
They indicate that Mr. and Mrs. Daan are not happy with the arrival of Mr. Dussel.
Step-by-step explanation:
When the dramatists claim that Mr. and Mrs. Daan murmured their welcome, they are saying that they welcomed Dussel in a very low tone of voice. That's because they were not being sincere about Dussel's stay, they didn't want an extra resident in the secret annex, because they believed that the coexistence was already bad with the residents that already existed there.