I've noticed this for a while now, and it's usually not a problem. As Skritter loads the characters it's getting ready for me to review, it plays the sound for them quietly. Usually it's too quiet for me to hear over all the other noise going on, but if I have my speakers turned up and the room is quiet it's definitely noticeable. Skritter apparently keeps four characters ahead of me, and faintly "previews" the sound for each character before I get to it.

Just now, I started a review session, and the first review was the Pinyin for the character 苗, which I got wrong the last time I reviewed it. But I didn't have a chance to think about it, because Skritter quietly "previewed" the sound as usual, and my speakers were turned up loud enough that it gave away the answer quite clearly.

Why does Skritter do this? Is there a solution in the works?

It's the ghosts. See: http://www.skritter.com/forum/topic?id=5677415 and http://www.skritter.com/forum/topic?id=93481562

I use to hear it but haven't for a long time. I think they've made it as quiet as they can.