Channel maps don't seem to be applying

[isXander]

Situation

I have a logical device with 4 channels.

SDL_AudioDeviceID device;
SDL_AudioSpec devSpec;

I have stereo sound data.

void *audio;
int audioLength;
SDL_AudioSpec audioSpec; 

I create an audio stream and bind it to my device.

SDL_AudioStream steam = SDL_CreateAudioStream(audioSpec, devSpec);
SDL_BindAudioStream(device, stream);

My aim is to play the stereo sound on channels 3 and 4;

 Input1       Input 2
   |             |
   |             X--------------------------X
   |                                        |
   X--------------------------X             |
                              |             |
 Output1       Output2     Output3       Output 4

...So I set the following output channel map:

SDL_SetAudioStreamOutputChannelMap(stream, new int[]{ -1, -1, 0, 1 }, 4);

I interpret this as;

• set output channel 0 to mute
• set output channel 1 to mute
• set output channel 2 to input channel 0
• set output channel 3 to input channel 1

I finally put my audio onto the stream:

SDL_PutAudioStreamData(stream, audio, audioLength);

Problem

Regardless of what I seem to try, the end result always ends up being;

  Input1       Input 2
    |             |
    |             |
    |             |
    |             |
    |             |
 Output1       Output2     Output3       Output 4