Answer:
the inability to be understood
Step-by-step explanation:
The word "muttering" refers to producing a long, dull sound, it also refers to speaking very low, inaudible and hard to understand.
The author in the above excerpt says that the retreats are muttering, that is, they are incomprehensible. For this reason, we can conclude from J. Alfred Prufrock's "The Song of Love" the word "muttering" refers to the inability to be misunderstood.