I assume your Slingbox is not sitting on top of any other electrical device, and its power supply is pluggd directly into a wall outlet, and not into any sort of power busbar. A low hum is often related to a symptom AC electrical interference with an audio circuit, which can generate a 60 cycle per second low tonal hum.
You might try connecting to your Slingbox via a web browser (IE or Firefox) at http://watch.sllingbox.com to see if that changes the hum, although I suspect that it is being generated at the physical source of your Slingbox and not through the software that you are using to view the streaming.
Good luck in your troubleshooting.
Are you using a cable box as the source feed? If so, make sure the cable box is set tto "fixed" audio output and not viable from within it's menu syastem options for sound.
When going though the slingbox setup you have an option to change the volume per input as well. Doing one or both of the following should clear up or at least inmprove the "hum" issue your experiencing.
Over amplification of a weak or low incomming audio signal gives you the symptoms your experiencing.
This post may be too late to help the person who originally inquired about steady audio hum, but this information may assist somebody who later makes the mistake that I did when re-connecting component cables after I moved a properly working installation. I erroneously plugged the red video input wire into the red audio rca jack, and likewise the red audio wire into the Pr component input. The result was a washed out video that lacked red chroma and the loud, steady audio hum. The audio problem was fixed when I unplugged the red video wire from the component audio and I had monaural audio with no hum. Switching the video and audio wires to their proper inputs restored normal video and audio.