Connecting subwoofer

What sort of inputs does your subwoofer accept?  Typically you can connect a subwoofer via either a dedicated LFE connection or you can run one of the front speaker channels to it and the subwoofer will only play frequencies it receives that are below the crossover frequency that you specify on the subwoofer itself.  The former is better if your Area 51 has an LFE audio output jack, but the latter will work too and is more widely compatible.  In the former case, make sure you actually configure your Windows audio settings to specify that you now have a subwoofer so that the LFE output actually gets used.  In the latter case, you won't have to do that because the front speaker channels will be receiving a full range signal, but to actually connect everything, you'd plug a 2-to-1 splitter into the front speaker audio output connector on the PC, then connect your actual front speakers to one output and your subwoofer to the other.  You might also need a 3.5mm to RCA adapter/cable for the subwoofer run.

@Ipearce1  in addition to my reply above, I just re-read your post and saw that you asked about using built-in speakers.  So does that mean you have an Area 51m (laptop) rather than an Area 51 (desktop)?  If so, then if you use an adapter to connect the subwoofer to the line out jack, if you see an option in the "What did you plug in" popup dialog that allows you to specify that you connected only a subwoofer, that might work.  Otherwise I doubt you'd be able to use the built-in speakers and an external subwoofer at the same time, although that probably wouldn't sound great anyway.  I would strongly recommend getting external speakers even if you only get relatively inexpensive ones.

@Ipearce1  ok, that extra info helps a bit.  In that case it sounds like your previous system allowed you to play audio simultaneously out of the built-in speakers and an external audio device (in your case a subwoofer) that was connected to the line out/headphone jack.  In all of the laptops I've used, if you connect external speakers to the line out/headphone jack, the built-in speakers are automatically disabled.  That always made sense to me because if you have headphones connected to that jack, then you wouldn't hear the built-in speakers anyway and in fact you might want those speakers muted if you're trying to keep things quiet for the people around you, and if you instead have external speakers connected, then you generally wouldn't want your built-in speakers also playing, because that would create a muddled audio experience for your ear.  I do realize that in your specific use case, that capability would be useful because your subwoofer isn't being used as a speaker in the traditional sense, but that's a very rare use case, so I'm actually surprised you had a laptop that allowed simultaneous audio output that way.

Unfortunately I can't think of a solution to this issue if you want to use the built-in speakers rather than external speakers (in which case you could use a 2-way splitter to connect both), and I actually hadn't even heard of a gamepad that implemented a rumble feature using an incoming audio and created the effect with a subwoofer.  The only way I've ever seen rumble implemented was with some sort of built-in rotor device (like phones have for their vibration feature) and then the game code actually supports a rumble feature, which is achieved by sending a rumble signal as data to the gamepad itself and therefore allows customized rumble patterns and levels independent of game audio.  Sorry!