Unfortunately it is not currently possible to play the Microphone input out of the speakers of a Dell Chromebook. There is no functionality within Chrome OS to allow such use of the inbuilt Microphone.
For reference, it is possible to play the Microphone input out of the computer's speakers in Windows 7 and later editions by:
Right-clicking the Speaker icon (in the Windows 7 Notification Area) and selecting the "Recording Devices" option
Selecting the "Listen" tab and checking the "Listen to this device" box.
The levels and sensitivity of this output on Windows can be adjusted accordingly if necessary.