The sound font has to be modified, at least a little bit.
Basically you have to create two more hum sounds, one with a slightly higher pitch and one with a slightly lower pitch.
One reasonable simple way to do so is to use a command called "sox"
sox hum.wav swingh.wav speed -180c rate -v
sox hum.wav swingl.wav speed 150c rate -v
This works, but the result isn't quiete as awesome as what Thexter was able to do.
More interesting results can be obtained by taking the hum from a *different* font to create the swingl/swingh sounds.