The most common and most likely answer to this crossword clue is the 4 letter word TRIG
T
R
I
G
More clues leading to the result TRIG
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 = trig.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 TRIG mean?
TRIG noun- A dandy; coxcomb.
- True; trusty; trustworthy; faithful.
- Safe; secure.
- Tight; firm; steady; sound; in good condition or health.
- Neat; tidy; trim; spruce; smart.
- Active; clever.
- Trigonometry.
- A trigonometric point, trig point.
- A stone, block of wood, or anything else, placed under a wheel or barrel to prevent motion; a scotch; a skid.
- The mark for players at skittles, etc.
- To stop (a wheel, barrel, etc.) by placing something under it; to scotch; to skid.
- To fill; to stuff; to cram.
- Triglyceride