If I decide to play a song in my head, I know that it's not real, but I don't hear that it's not real. Not because of the quality of the rendition, but because it would be indistinguishable to me from hearing an identical live reproduction [1]. If I'm really tired and start having sensory hallucinations, I don't know that they're not real and I don't hear that they're not real.
Personally, after experimenting with it, I believe that when I'm imagining sights or sounds, I'm actually self-inducing sensory hallucinations. I can do it with other senses too, but I have less practice with those. I don't have to visualize in order to think or conceptualize [2].
All of that is what makes me think that aphantasia is the inability to induce sensory hallucinations at will. Depending on the person, it might be limited to one or more senses, the quality might be limited by available bandwidth, but if it's visualized then it would not be distinguishable from an equivalent sensory input. If it's conceptualized, then by definition it's not a sensory input. Of course, I only have my own subjective experience as a data point, so I don't expect this to be an authoritative answer.
[1] Excluding other correlated stimuli, like feeling my innards vibrating due to proximity to a speaker emitting a loud base.
[2] I'll have to think and experiment on my own to quantify this part. That being said, I haven't managed to read a sentence without having a voice speaking out the words in my head.