There's another solution that works, but is even less practical. Get a soundcard with optical input. Connect one end of the optical cable to the console and the other end to the soundcard's optical in. Connect the speakers to the soundcard analogue out. The sound will be output to the speakers using the sound card. Like I mentioned, it does work, but this is hardly a good solution because:

1. You need to buy the soundcard, which won't be cheap (X-fi Platinum or similar)
2. The PC needs to be turned on while playing
3. Can't use this setup when connecting to a television set (unless the PC is dragged to the TV)

This brings us back to the most proper solution: buying the decoder. I've been contemplating buying it from 2003-2005. But I found myself resisting because of the high asking price. In the end, I just settled buying a Logitech Z680. This ended up being my gaming setup until now. I can't say whether the price of the decoder is worth it, but it really is the most proper solution to your issue.