From pg. 110 of Volo's Guide to Monsters, under the "Hopeless Plagiarists" heading:
Although unable to speak in their own voices, kenku can perfectly
mimic any sound they hear, from a halfling's voice to the noise of
rocks clattering down a hillside. However, kenku cannot create new
sounds and can communicate only by using sounds they have heard. Most
kenku use a combination of overheard phrases and sound effects to
convey their ideas and thoughts.
From the description given, kenku should be entirely able to isolate partial sounds if they can string them together in combinations. The largest factor in being able to mimic their friend's conversation would be in hearing clearly enough to recognize the individual noises as words - if it was a language they did not understand, it would carry no more meaning than the clattering of rocks or the drip of water, and this the kenku could not deviate in volume or tone or speed from how it was communicated originally as to do so would be to create new sounds by way of editing old ones.
If it is a language the kenku understands and it's reasonably clear but there are some parts they can't share well - such as whispers - they should be able to substitute any individual words in the statement for the same word it had heard more clearly elsewhere, but this obviously carries with it the drawback of not being in the friend's voice.