When we speak, our ears give us feedback on the same. This, presumably, helps in learning a language and adjusting the volume of our speech. How do deaf people get this feedback to learn a new language or adjust the volume of their speech?
Many "deaf" individuals still have some functional hearing. Even in the case of a total loss of hearing, there are proprioceptive cues that provide useful feedback for speech production. The typical way of teaching pre-lingually deaf children how to speak involves hours of one-on-one time with a speech-language pathologist. The child learns to speak by placing their hand on the throat/mouth of the instructor and feeling the vibrations and movements of the articulators. There is also software that process speech in near real-time and helps the talker adjust their speech to match a target pattern. These same techniques could be used for the post-lingually deaf. That said, I think it is pretty rare for post-lingually deaf individuals to learn a second spoken language.