Alienware 18 speaker not working only headphones

Hello. You can test the speakers by performing the Quick Audio Check in the ePSA test. If the speakers fail the test then they have failed or become disconnected.

If the speakers pass the test, then test the audio driver by switching between the Realtek audio driver and the Windows native driver.

1. Open the Device Manager (find it in the Control Panel, or type devmgmt.msc into the search box).
2. Expand the "Sound, video & game controllers" and right click on "Realtek High Definition Audio".
3. Select to "Update Driver Software".
4. Click on "Browse my computer for driver software".
5. Click "Let me pick from a list of drivers on my computer".
6. Put a check in the box "Show compatible hardware" if not already checked.
7. In the list of devices, click "High Definition Audio" (the native driver).
8. Click "Next".
9. On the Update Driver Warning box, click "Yes" (install the driver).
10. Restart the laptop if prompted. If not prompted, then no need to restart.
[To get back to the Realtek driver, do it again but reverse the names in steps 2 and 7.]

If the speakers work correctly with the native driver, then the Realtek driver is buggy. If the speakers do not work with either driver, then it is not a driver issue. It could be that the sensing mechanism in the headphone jack has failed, making Windows think that there is a plug in the jack when there is not. More details in the Headphone Jack FAQ.

when i open the playback devices it shows sounds are coming out of speakers whether i got my headphones plugged or not

If you mean that only "Speakers" is listed as an analog playback device, that is normal when a Realtek is installed. It won't have a separate listing for headphones.