Both are used, but not in the same way.
The answer is sound out.
I am new in this forum and just found phrasal verbs thread. I think the way questions being asked here is excellent. I really like these thread. Thank you so much for the questions, Tdol.
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]