Both are used, but not in the same way.
The answer is sound out.
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Definitions according Cambridge Dict. :
sound out sb/sth : to talk to someone in order to discover what they think about an idea or plan
I sounded her out about working for us but she wasn't interested. [often + about]
We need to sound out his views on the project before we go ahead.
sound off : to express your opinions loudly, especially in order to complain about someone or something
He's always sounding off about the way this place is run. [usually + about]
Last edited by Siregar; 27-Feb-2007 at 22:22.