Can I enable my Realtek sound card without speakers plugged in?

Solution 1:

Disable automatic jack detection by following the instructions in this thread. Relevant excerpts:

Open regedit with administrator privileges
Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class{4D36E96C-E325-11CE-BFC1-08002BE10318}\0000\GlobalSettings
Change the value EnableDynamicDevices to zero.
Reboot

EDIT BASED ON COMMENT:

If the registry approach doesn't work, you have two other options:

Set the option in the Realtek Control Panel (should be available with latest Realtek drivers for the sound card). Step by step details with images can be found at this website. Relevant excerpts:

Go to Control Panel -> Hardware & Sound -> Realtek HD Audio Manager
Click the yellow folder icon in the top right of the window
Disable front panel jack detection and auto popup dialog, click ok.
Click on device advanced settings at top right of window
Check the buttons for make front and rear output devices plaback two different audio streams simultaneously and tie up same type of input jacks..., click ok.
Open the speaker tab
Click on the green active speaker icon (should be under back panel on right side)
Click set default, click ok.

Replace the Realtek drivers with Microsoft HD Audio driver. Details can be found at this website. Relevant excerpts:

Uninstall the Realtek audio drivers and reboot
Open the device manager
Under Sound, video and game controllers, right click on High Definition Audio Device and click Update Driver software
Click Brows my computer for driver software
Click Let me pick from a list of...
Choose High Definition Audio Device
Click yes when warning pops up.
Finish confirming dialog windows and reboot

If for some reason you can't disable the jack detection via software, keep in mind that the detection is done by a mechanical switch inside the jack.

Cut the plug off a set of unneeded headphones, and plug it into the jack to activate the switch.