How to mute internal speaker of Dell Optiplex 990 so that audio plays via external speaker only

After upgrading Ubuntu 12.04 (Precise Pangolin) to 12.10 (Quantal Quetzal), the computer started playing sound through both the internal built-in speaker and external jacked in speakers. To mute the internal speaker, run:

/usr/bin/amixer -c 0 sset 'Auto-Mute Mode' 'Line Out+Speaker'

Just make sure “Auto-Mute Mode” is not set to “Disabled”.

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