[ElDiabloConCaca]

The Apple Pro Speakers use a proprietary connection that resembles a 1/8" headphone jack plug, but it isn't.

You're not gonna get what you want out of this. The Apple Pro Speakers must be plugged into the Apple Pro Speaker port on the computer, and any 1/8" plug devices you have must be connected to the 1/8" audio-out port of the computer.

The reason is that the Apple Pro Speaker port is powered, while the 1/8" audio-out port is unpowered. That's why you don't have to plug the Apple Pro Speakers into any kind of power, while you must plug in any powered speakers that you use with the 1/8" audio-out port (or suffer through barely-audible sound).

As far as I know, there are no converters that convert from one plug to another, although I'm sure one could be fabricated -- I wouldn't attempt it, though, unless you're skilled with electronics (and I don't mean you have no trouble programmign a VCR) and know the ins, outs and inner workings of both of those ports.

I would suggest ditching the Pro Speakers altogether, then splitting the audio out of the 1/8" jack as suggested.