As you know, Audio Interfaces have separate Stereo output channels:
It would be great if Splash could map each Stereo outputs for full 5.1/7.1 surround format
Here's my audio interface that has 3 stereo outputs that is capable of 5.1 surround:
I think this might depend on the audio interface more than the player. At least my Echo AudioFire4 seems to think it's a 4 channel audio card in many games if it's not set to separate stereo output mode.
I doubt Splash has anything to do with this. The output hardware (your soundcard/audio interface) is responsible for this)
If you are running analog outs, use high quality signal splitters to your AI and out to your amp. Splash is simply software that processes the audio/video and sends it out to the hardware. How its dealt with after that is upto the hardware.