Alejandro,
We use the built-in Modal/Squeezebox Javascript library that's a part of Joomla. From a quick look, unfortunately, it looks like the Squeezebox position can't be set. The size and other options are available, but the position seems to be fixed at 50% in both the height and width of the view. That normally makes sense because the whole background is also shaded, so you want the user's attention drawn right to the middle of the screen.
I'm not sure how you'd be able to set the position, but hopefully the explanation helps you understand how it works a little more.
Sorry I can't help further, but let us know if you still have questions. We can try to look into it further, but it doesn't look possible.
Thanks,
Alex