Title. I predict this may be a side-effect of whatever scripting was done to make subsonics apply bard song effects "quietly", but as a damaging attack effect, Curse Song should break invisibility when used, and its damage should, similarly, appear in the combat log (for the benefit of both the bard, and whomever was assailed).
As it is now, I can tromp through wherever I please using Move Silently and Curse Song, and it does not break invisibility (nor does it even cause enemies to detect me).