The most common and most likely answer to this crossword clue is the 3 letter word MEH
I've seen this crossword clue in The Guardian
M
E
H
Buddy explains!

The straight part
We know that "Sound of indifference/Sound of indifference" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sound of indifference/sound of indifference = meh.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MEH mean?
MEH noun- A judgement marked by indifference; lack of impression.
- Mediocre; lackluster; unexceptional; uninspiring.
- Apathetic; unenthusiastic.
- Expressing indifference or lack of enthusiasm.