I have used FastScripts to assign a shortcut to this script:
set old to the clipboard as record
tell application "System Events" to keystroke "c" using command down
say (the clipboard) using "Kyoko"
set the clipboard to old
You could also create a service in Automator:
There is a bug in 10.7 and 10.8 where the shortcuts for Automator services don't always work until you hover over the services menu from the menu bar. WorkflowServiceRunner can also use over 100% CPU while speaking text.
Another option would be to use UI scripting to change between two voices:
tell application "System Preferences"
reveal anchor "TTS" of pane "com.apple.preference.speech"
tell application "System Events" to tell process "System Preferences"
tell pop up button 1 of tab group 1 of window 1
if value is "Alex" then
click menu item "Victoria" of menu 1
click menu item "Alex" of menu 1
quit application "System Preferences"
Changing the SelectedVoiceID key in com.apple.speech.voice.prefs.plist also works, but I don't know how to apply the changes immediately.