The most common and most likely answer to this crossword clue is the 3 letter word BEL
I've seen this crossword clue in Independent
B
E
L
Buddy explains!

The straight part
We know that "sound" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sound = bel.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 BEL mean?
BEL noun- A measure of relative power, defined as log10(P 1/P 2), where P1 and P2 are the measured and reference power respectively.
- A tropical fruit tree from India, Aegle marmelos.
- The fruit of the tree, also called the wood apple.