When using a Pocket PC to control iTunes you can select the AirTunes speakers used for audio playback from your device. From the main control screen select Menu > Speakers to view the list of available speakers. Tapping on the name of a speaker will redirect audio to that speaker.
You can also control synchronized audio playback through multiple speakers by checking the "Use Multiple Speakers" option that appears on this menu. When this option is checked tapping on the name of a speaker will toggle that speaker on or off.