RE: No Headphone or Speakers plugged - Though I have Internal Speakers
Welcome to the Dell Community Forum,
Please check in Device manager under option Audio if you notice any exclamation mark / question mark. Try re-installing the audio driver from Dell's support site here http://dell.to/1hcxG98, type in the Service Tag, select the OS type and install the Audio drivers.
Complete the same, restart the computer and let us know if there are any other issues.