Do you have SI|3D too? it has channels to read in sound files and output to an channel of an object producing an fcurve.
Or use another 3D package to convert sound to curves and export the curves and import into XSI. Maya has one now right?
I prefer Houdini as it has a full Channel Operator interface with OPs to filter, convert, resample etc, the sound before exporting to an animation curve, then export its curves or the fastest most relaible but admitedly worst way to get it into other packages is to plot the cure to a dxf and import that into the other package and convert it to a curve or put sometng on it as a path and plot. The most accurate way is export the curves and import them.
XSI definitley needs more channel development.