I have music in my head 99.999% of the time and it’s not always something I recently listened to. I usually wake up with a random song in my head and it’s in there until I put on music for real, otherwise it sometimes switches. It’s usually a song I’m familiar with and like. Sometimes I only hear the vocals if the instrumental backing is too complicated and sometimes it turns into a medley of two songs. It’s not as clear as if I were actually listening to it, but clear enough that if I’m stuck somewhere without an ipod and am enormously bored I can entertain myself by “listening” to what’s in my head and seeing if I can remember all the lyrics and “play” the song in its entirety.
I mentioned this to a friend and she was perplexed saying that she only gets songs stuck in her head if it’s something she recently heard and then it’s only for a few moments.
My mom has mentioned “hearing” songs, but in her case it’s just vague background noise and she can’t make anything out. She has temporal lobe epilepsy though, so that probably has something to do with it.