I have had Machine Head on the background while working this morning and the audio quality sounded surprisingly good for a webcast. Slayer is taking the stage within the next five or so minutes. (If you are in the US, ignore that the site says Shadows Fall is playing next. The site isn't accounting for time zone differences). Here is the URL for the webcast. It looks like Slayer is the last band to be webcast today as there is no webcast icon next to the final two acts (Marilyn Manson and Linkin Park).

The last day of the Download Festival is tomorrow and I would assume most of it will be webcast.