It appears to have become a "hidden preference" in later versions of iTunes. Using the Terminal commanddefaults write com.apple.iTunes play-songs-while-importing -bool TRUE
worked to restore this pref at some point, although cursory googling indicates that that may no longer be the case. (I'm unable to test the command myself because my current setup doesn't include an optical drive.)
If you're disinclined to try the Terminal approach, Doug Adams has an AppleScript script available—Change Hidden iTunes Preferences
—which numbers among its invocable prefs "Play songs while importing or converting." This script carries the following note: This script has not been updated for iTunes 11. Several settings* are no longer applicable in iTunes 11,
but the preference in question is not marked as one of them.
My guess is that the script is just a front-end to the Terminal command anyway, so it might be more straightforward to use Terminal directly. In that case, you'd run the command with iTunes not
running; it probably wouldn't be a bad idea to restart, as well, since some pref changes don't seem to "take" if you're only relaunching the app.