There may be no (direct) common reference between the two people, but there is a common reference. If a person hears a sound, and then later their mind reproduces that sound (say, remembering a song) they'd say they 'hear' the sound in their head (or at least I've never met anyone that doesn't unless they're making a fine distinction between the subjective experiences of hearing with the ears vs 'hearing' with the mind.) If someone says 'I don't hear my own thoughts/memories', particularly in reference to a common experience (take the song example from earlier) we can reasonably infer that their subjective experience is very different from someone who does 'hear' their own thoughts.

I know that personally, I am quite capable of hearing (or seeing) thoughts/memories, and the experience is near enough to the real thing that I wouldn't use any other term other than to clarify that I'm not undergoing some sort of self-induced hallucination.