It is not a free plug and it only accepts sound font files.
The sfz.exe is free and vastly more accessible.
Unfortunately sfz.exe seems to share memory between instances in some way and some folks had difficult-to-reproduce Sonar-killing crashes when more than one is in a project.
There are improved sfz file players out there, but I'd love to see the sfz.exe fixed. DimPro is basically 4 sfz.exe with yet more (redundant but accessible over a GUI) functionality. I have not checked how much of the sfz file spec DimPro covers. I really should take the time some day.