I recently found that the microphone stopped working, although the internal and external speakers worked correctly.  I visited Dell support to see if the Realtek driver needed to be updated.  It reported thast all drivers were up to date.  Nonetheless I downloaded the Realtek Audio driver (Audio_Driver_PKGMR_WN32_6.0.1.7564_A01.EXE) and ran it in case the existing driver had become corrupted.  Hey presto! the microphone then worked, as did the internal speakers.  However, when I plugged in external speakers they just gave a combination of white noise and mains hum.

I have tried numerous versions of Realtek audio drivers and can't get the external speakers to work with any of them.