Whenever I have a hearing or Deaf or HoH passenger, I turn off my radio/CD player. It's just common courtesy.
Unless your passenger requested a serenade or karaoke performance, you should probably restrain yourself. It's not so much whether or not the passenger can hear you sing. It's the idea that your attention (other than what's needed for driving) should be towards your guest. Even if you can't sign together, an occasional glance, nod or smile towards your guest to acknowledge his/her existence would be nice. 
