Actionscript Code:
var depth = _root.getNextHightestDepth()
_root.attachMovie("sol1", "sol1" + depth, depth, {_x:_xmouse, y:_ymouse});
Otherwise you'll only be able to add one movie. Because multiple movies cannot occupy the same depth, when a second one is added, neither will show up. Also to be able to access the dynamically added movies from other code, they need to have unique names. Using getNextHightestDepth like this solves both these problems.
Actionscript Code:
var container = _root.attachMovie("sol_container", "sol_container", 1);
var depth = container.getNextHightestDepth()
container.attachMovie("sol1", "sol1" + depth, depth, {_x:_xmouse, y:_ymouse});
Alternately, you can use do your own depth tracking and make sure that each added movie gets it's own depth and name, but remains below other items that it's supposed to remain behind.