5.1 sound from Xbox 360 through 'computer speakers 5.1' set?

[Cretster]

Surround from 360 through 'computer speakers'?
 
My arcade machine has an audio conundrum.
 
It's fine for sound from the PC where the soundcard can output signals for 5.1 channels via RCA plugs, and these go directly into the Amp/sub thing (cheap computer 5.1 speaker set).
But my Xbox 360 can output HDMI, (where I can't get the sound signals to the sub/amp). OR, with an additional cable it can give me stereo RCA out + optical digital out.
 
Now the sub/amp/speaker thing has no optical input, so I can't do that.  And if I use the stereo RCA outputs I won't get 5.1 sound, just cacky stereo for the games.
 
My PC doesn't have an optical input so I can't feed the signal into that and back out with the rest of the sound.
 
Help me obi wan audio boffins - you're my only hope!
 
I could use a spare full size AV amp I've got, but too bulky and then I'd also need a powered separate subwoofer as well.
 
So what's the (cheap) solution here for surround from the 360?

[BadMouth]

I don't have any experience with an xbox360, but I can tell you that I've been in HDMI "handshaking" hell with my home video distribution setup.

My laptop says it detects a repeater inline, so other devices can as well.
My home theater receiver will not play in 5.1 surround if HDMI passthrough of audio is enabled, only 2 channel.  (It will not allow 5.1 to be passed through it to any other device)
There are a ton of other issues I had to overcome, but they aren't relevant to yours.

What I'm getting at is that using a device that breaks the audio out of the HDMI signal might lead to more headaches.

If the xbox360 has an optical or coax digital audio connection, I'd use that with a standalone dolby digital decoder.
Quick search turned up this one for $50, no idea of quality: http://www.amazon.com/Premium-Audio-Digital-Decoder-Converter/dp/B00NBDKQSY/ref=sr_1_3?ie=UTF8&qid=1423670931&sr=8-3&keywords=dolby+decoder+analog

If it doesn't, then get a decoder that will downmix the surround sound from standard stereo RCA inputs.
It won't be quite as perfect, but I bet it will be close enough to make you happy.
EDIT: looks like the one I linked does that from the aux input

[BadMouth]

If you just want some sound coming out of all the speakers, my Logitech 5.1 system came with one of these:


I've never used it, so can't say how it sounds.  I can't imagine they cost more than a few bucks though.

[Fursphere]

I was looking at this same question not too long ago, but for my PS3. 

The solution I came up was was this:

Logitech (or Creative Labs) 5.1 speakers with Optical connection.  These are pricey.

Creative Labs Sound Blaster Z (already have one in my driving cab).  It has separate optical input and outputs.

Run PS3 optical audio out into sound blaster optical audio in. That should overcome all the issues om theory.  Except its expensive.    And it's assuming you want both a console and a PC in your setup.