The web player has a link under the Watch tab for the "Slingbox Directory" but there are no options to adjust audio/video as you can do in the standalone player.
I'm not on my local network right now, which means I can't access the "Setup Internet Viewing" option in the web version of the Slingbox Directory. If you are on your local network (i.e., home), try looking in there, but I'd imagine this is for setting up the network and not for any A/V settings, meaning you may be out of luck.
I checked my iPhone as well and it has no way to do this either.
It seems to be just one more reason the desktop player beats this web-based junk hands-down (and with the Mac player discontinued after NEVER having had HD support, Mac users really get hosed).