5.1 sorround sound

which player you use?

Are you sure the test video you are trying to play supports 5.1 channel sound? It may be your video is playing stereo only. Verify the video offers 5.1 surround sound and that has been selected.

The cable looks correct, but verify it supports a Toslink 5.1 optical connector and a mini optical connector for the iMac. This has nothing to do with MIDI. Perhaps you are not connecting to the correct ports.

Plug in the cable to your Pioneer system and ensure the Pioneer system is powered on. Plug into the iMac’s output port (the port with the headphones icon). Open System Preferences/Sound and click on Output tab. You should see an entry for your line-out. Select that entry and verify the other sound settings are appropriate.

try to reconnect cable and in VLC preferences select passtrought options for DTS and DD

and try to play some DVD movie

I managed to get your 5.1 test file to play through all channels once I made sure that in VLC under the Audio menu on the title bar that Audio Device was set to Built-in Output (Encoded output). The weird thing is that option doesn't come up unless you are playing the track.

I don't really know what you mean, but I have to say this is all very confusing.

I can select 'Encoded Output' in VLC, and everything is OK, but whenever I quit from it, and reload, it goes back to the non-encoded output, and has to be reset again during play.