Neither one of those links works.
It kind of boils down to this. If your speakers have only unbalanced RCA inputs, then you'll probably do better to just get that adapter at radioshack unless you want to hunt for an audio interface that also has unbalanced outputs (most of them don't). Otherwise, you'll only be using the interface to record with and/or use headphones with.
If your speakers have both inputs (RCA and 1/4" jack) then I would go ahead and get the interface and use the balanced outputs on it and run it out to the speakers. Audio interfaces are just one of those basic things that every home recording musician should have just in case. I went a long time without one myself and once I finally broke down and shelled out the cash, it made my life a lot easier in the rare instance that I had to record something live (most of my compositions are MIDI based too!). It was nice to have that weight lifted off of my shoulder.
***Please note that once you connect the audio interface to your computer,
all of the computer's sound will be handled by the interface. This includes safari and iTunes. All volume adjustments are also handled by the interface. In fact, the on-screen volume controls on your desktop become grayed out once the interface is connected (and selected under your system sound preferences of course).***
I am attaching some pictures of my monitors and audio interface to help illustrate these connections.
Back of one of my monitors (KRK Rokit 5's). Notice the RCA and 1/4" jack inputs.
The 1/4" jack balanced connection
Back of my USB audio interface (Alesis iO2 Express). The 2 black circular connections on the right are a MIDI input and output. Notice it does not have any RCA outputs.
and the interface.
I'll explain as they all pretty much function using the same principles.
-The main level knob (bottom right) is just the main volume knob for the monitors.
-The monitor mix knob adjusts the volume of your system audio relative to what the microphone is picking up. Turn it up and you'll get just your music and no mic audio. Turn it down and your music gets quiet and the mic audio gets loud. If it's turned down and you talk into a connected microphone, you will hear yourself on the speakers. If it's up, you will hear nothing when talking into the mic.
-The knob next to the headphones input (bottom left) adjusts the main headphones volume.
-The 2 gain knobs right under the mic, line, and guitar inputs adjust the preamp volume on the microphone. Turn them up and your voice will be louder in the recording.
Hope this was helpful.