I just went through the fun of getting this to work on Debian Squeeze with my revo. You need an .asoundrc and in there mine is set to plughw:1,7 . Also in boxee then you go to settings > sound > and set the output to default. I'll post a copy of my asound which may work for you.
P.S. I also noticed that the default libflashplayer.so that ships with boxee does not have vdpau support. So full screen flash plays like garbage on ION and ION2 systems untill you replace their /opt/boxee/system/players/flashplayer/xulrunner-x86_64-linux/bin/plugins/libflashplayer.so with an updated one. Basically just sudo apt-get install flashplugin-nonfree and copy the Ubuntu or Debian one from /usr/lib/flashplugin-nonfree/libflashplayer.so to the /opt/boxee........ one.