I've had my M18x for only 3 days now. I decided to be adventurous and attempt S/PDIF with my home theater surround sound. Boy was that a bad idea. The S/PDIF connection worked great, although the connector fit a bit snugly. When I decided I wanted to disconnect it, it came out not so easily. I didn't twist or bend it, I pulled it straight out. I attempted to reconfigure the sound settings to default back to the laptops on-board speakers. Low and behold, all I got was a popping noise. I can recreate it simply by pressing mute and then off again. I've seen a few videos of individuals having this same issue occur. Mostly the repair guys that get called out are told to replace the speakers themselves. I however, know that this is not the case. The reason why? S/PDIF connection is no longer recognized by the system. Sounds like a motherboard short to me. Very isolated, though I must say. I may attempt to connect my via HDMI-In just to rule out the speaker issue. HDMI should not be affected and should output sound like I expect it to.
I consider myself pretty lucky to have this be the only issue. I probably will contact Dell tech support in the next few days and see if I can't get the mobo replaced. Ultimately though, I am not concerned as long as the S/PDIF & On-board speakers are the only devices to cut out considering I wasn't too impressed with the on-board speaker sound (Klipsch...pfftt...more like Glipsch) as it sounds like its down-firing. I've ran all the diagnostics that have been recommended, but I know a faulty motherboard when I see one. Clearly it wasn't meant to handle the stress of S/PDIF. Others have reported similar glitches with a non-usb headset. They would finally decide to go back to using the on-board speakers and they just wouldn't work.
I was planning on purchasing some portable speakers to go along with this laptop (Razer Ferox is one I had in mind). Ultimately I would be using a headset of some sort (Tt Sport Isurus comes to mind) and preferably one that goes in the ear as opposed to around. I know the M18x isn't the most portable beast, but at least its not like trying to move a desktop to and fro. I even managed to find a USB TOSLINK/S/PDIF connector (LOL) for $29.99. So I am not too worried if when my laptop comes back, nothing changes. I'll probably just forgo attempting on-board S/PDIF ever again and just focus on the USB one and either of the two solutions I mentioned above. Of course there's always the chance my speakers could cause the controller to blow by themselves...
Note: Alienware also claims that the S/PDIF port can be used as an ordinary headphone/speaker connector port. I connected a headset and got nothing. Very convinced that this issue has to do with the motherboard!