I have no idea what could have caused this. About a week ago the output volume on my mini halved, I used to keep it at around 50% but now it needs to be at 100% for the same volume. It hadn't been synced for a week prior to this, so it wasn't iTunes. There doesn't appear to be a volume limiter in iTunes for the mini (unlike the shuffle) or one in the iPod's settings. It's definitely the iPod and not the earphones I'm using as I have an identical pair and the volume is half on those too.