I have a fix for that. If I can find the thread that describes it, I'll post it.
Basically, you take a section of the plastic strip that comes with a glass top/canopy and use it as a lip "extension" for your HOB filter.
First, cut it to the width of the filter's output. Depending on the manufacturer, it will require regular scissors or something more heavy duty. Next, dip it in hot water to make it slightly flexible. You then bend it (an "S" curve, for instance) to suit the needs of your tank, adjusting for the drop or distance between the filter and water level. You could bend a lip at the end so that it runs parallel with the water surface.
This accomplishes a few things: reduces the noise associated with splashing and it directs the flow along the surface (or whatever angle you set it at). This will reduce plants getting pulled down by the waterfall and disruption to the substrate in some cases.