What part is this?

Okay, so I have multiple devices (computer + USB interface) that I want to connect so that they share the same set of 2.1 speakers. Here's a simple little diagram:

Computer --------
\
-- Speakers
/
USB Interface ----

My 2.1 speakers only have one 1/4" male plug to use as an input. So, I tried getting a simple splitter (similar to the one in the link below), but every time that I plug both inputs into the splitter, there is a sudden and drastic drop in volume. However, whenever I take one of the inputs out of the splitter (so that only one of the connections is being utilized), the volume returns to normal levels. I was kind of looking at something like this stereo headphone amp (http://www.amazon.com/Behringer-HA400-4-Channel-Stereo-Headphone/dp/B000KIPT30/ref=sr_1_6?ie=UTF8&qid=1350623729&sr=8-6&keywords=1%2F4%22+input+port), but I would really need the opposite of what it provides...multiple inputs and one output.

Any ideas of what I can get so that I can use the same set of speakers for both devices? As much fun as it would be to go out and buy new speakers (cough* toys!! *cough) I'm hoping that I can avoid having to do that.

Thanks!!!!

Link to similar splitter: http://www.amazon.com/Belkin-Speaker-and-Headphone-Splitter/dp/B000067RC4/ref=pd_cp_e_1


Note: Sorry the diagram came out kinda weird for some reason. If you click on 'quote' it should show up correctly

You need a mixer. There are a bunch of low cost solutions from alesis & behringer.

You didn't elaborate on if these where mono or stereo audio sources, keep that in mind when you select a mixer so you get one with the inputs you need. .

Dom