Ricven mentioned parts of the old weave which allowed this old spell to work did he?
Hmm, I happen to be in possession of an old map of arcane power concentrations in the area. My theory is that the most concentrated spots would remain stable enough to allow you to cast ghost sound. Here, I'll mark the locations on your map.
While you are rummaging about out there, keep your eye out for any old tomes. Who knows what other spells we might find!