Notification Center "Tweet sent" bird whistle plays through iMac internal speakers instead of selected output device Tweeting through Notification Centre share buttons plays "Tweet sent" bird whistle through my iMac's internal speakers. This only happened when I re-enabled the option to select internal speakers themselves by unplugging the speakers, installing more RAM, restarting the computer and plugging the speakers back in.

Before this, Mountain Lion played all sounds through my headphones; the selected audio output device in System Preferences.app > Sound. Now all sounds are being played through the output device as they always did—except for the "Tweet sent" sound—definitely not what I want.

How can I get it so the "Tweet sent" sound plays through my headphones again?

This also applies to other system/alert sounds it seems, such as the set alert sound, emptying the trash.
Open System Preferences
Go to Sound -> Sound Effects Tab
Select "Play sound effects through selected sound output device"