Dell XPS L502x JBL Subwoofer not working

Last year i upgraded it to Win 8.1 Pro OS.  Post that, I'm having issues with the JBL system. Thoguh the Front speakers are working perfectly, the JBL subwoofer on the back are not working.

Hello. If the woofer worked for a while after installing Win8.1, but then stopped working, it could be hardware failure. The woofer was prone to failure on that model. The solution would be to replace the woofer.

If it never worked at all after installing Win8.1, it could be a driver issue. Dell supported Win7 and 8 on the XPS L502x, but not Win8.1. It might help to install the Win8 Realtek audio driver in compatibility mode. First remove the current Realtek driver and delete its files from the hard drive, then

1. Download and save the driver to your desktop or any convenient location on the hard drive. 
2. Double click on the new folder to extract (unzip) the driver files.
3. The extractor wizard will create a new folder for the driver files. It will be at "c:\dell\drivers\xxxxxx". Write down the exact location that the wizard creates.
4. If the driver begins to install automatically, halt (cancel) the installation.
5. Browse to the driver files on the hard drive (the location you wrote down).
6. Find the "setup.exe" file.
7. Right click on setup.exe to open the context menu.
8. Select the Properties.
9. Select the Compatibility tab.
10. Check the box "run this program in compatibility mode for"... Win8.

couldn't get the Soundblaster driver for Win 8.1 for this model.

I don't think Dell released any Soundblaster driver for the L502x, regardless of operating system.