There is a much simpler and cheaper solution to the problem. Use an inexpensive audio board to feed your speakers. You can still use firewire to connect your deck to your computer for audio and video capture connections. Instead of feeding the audio signal directly from your computer to your speakers, go directly to the audio board and then to the speakers. So now you will be coming out of the computer and going to the audio board and then to your speakers (if you are using powered speakers). Now use the audio from the audio/video connectors on the deck and connect that to two additional channels on the board. When you are capturing raise the faders on these channels and you should have audio from the deck playing through your speakers. Mackie, Behringer as well as other manufacturers make excellent small boards that will work well for this purpose. You can frequently score some of these boards cheaply on eBay as well.
Another option is to use a splitter on the speakers and connect that to your deck and to your computer. But you will not have any control over the volume of the signal coming from the deck.
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