For my favorite songs, I can recall it perfectly... But I don't hear it at all as if I was hearing it in real life. In fact, I don't know how to describe how vivid it is. Maybe I don't hear it at all? But I can sing these songs (out loud) without a problem.
However, I seem to be incapable of changing some elements about it. I can't seem to be able to make it faster, slower, change the singer, or the instruments. But, when I sing it out loud, I can of course modify it.
For "Happy birthday" (well, in my native language), it's really weird. Because I don't have a specific song in mind, I can follow the words, the tune, but I don't "feel" the voice of a specific singer. And I'm incapable to change this recall to different voices, like children or grown men. But if I listen to a version sung by these people, I can recall it after (until I forget).
Thanks again for these questions, I was aware of my lack of inner voice, my difficulties of a "inner sight" (that's a whole other can of worms), but I never applied this interrogations to a "inner hearing".