Saying that someone is deaf, is like saying that someone is European or British. It doesn't tell you very much because there are major differences between subgroups.
Not all deaf people use sign language. Someone who becomes deaf later in life, may not learn sign language. Someone who was raised and educated orally may prefer to communicate using lipreading and speech.
Some deaf people can hear - with hearing aids, or Cochlear Implants. They may be able to use the spoken language for communication, but usually only under optimal circumstances: someone who speaks clearly and not too fast, one person speaking at a time, and no background noise.
Some deaf people are comfortable speaking. Others may not want to use their voice.
Don't assume that a deaf participant knows sign language, but ask what his/her preferences are.