AudioUnit Render callback issue (callback not called)

I have a simple requirement at the moment - an iOS app that reads from an audio file and outputs to the speaker using AudioUnits. The reason for not using the high-level APIs is that at some point I need to process the samples coming out of the audio file and eventually send them over the network.

I have some code that works, reads an audio file and plays it back. The only problem here is the render callback doesn't work. The callback is never called and I don't get any errors on registration. Help is greatly appreciated (I'm new to Core Audio and this is my first stackoverflow question, so please apologize for any major mistakes / shortcomings). The piece of code I am using to initialize the graph is attached.

void createMyAUGraph (MyAUGraphPlayerST *player) {
    // Create a new AUGraph

    CheckError(NewAUGraph(&player->graph), "New AUGraph failed");

    // Generate description for output

    AudioComponentDescription outputcd = {0};
    outputcd.componentType = kAudioUnitType_Output;
    outputcd.componentSubType = kAudioUnitSubType_RemoteIO;
    outputcd.componentManufacturer = kAudioUnitManufacturer_Apple;
    outputcd.componentFlags = 0;
    outputcd.componentFlagsMask = 0;

    // Add new node

    AUNode outputNode;
    CheckError(AUGraphAddNode(player->graph, &outputcd, &outputNode), "Add output node failed");

    // Node for file player

    AudioComponentDescription fileplayercd = {0};
    fileplayercd.componentType = kAudioUnitType_Generator;
    fileplayercd.componentSubType = kAudioUnitSubType_AudioFilePlayer;
    fileplayercd.componentManufacturer = kAudioUnitManufacturer_Apple;

    // Add new node

    AUNode fileNode;
    CheckError(AUGraphAddNode(player->graph, &fileplayercd, &fileNode), "Add file node failed");

    // Open graph

    CheckError(AUGraphOpen(player->graph), "Graph open failed");

    // Retrive AudioUnit

    CheckError(AUGraphNodeInfo(player->graph, outputNode, NULL, &player->outputAU), "file unit retrive failed");
    CheckError(AUGraphNodeInfo(player->graph, fileNode, NULL, &player->fileAU), "file unit retrive failed");

    // connect nodes

    CheckError(AUGraphConnectNodeInput(player->graph, fileNode, 0, outputNode, 0), "failed to connect nodes");

    // some other setup

    UInt32 flag = 1;
    CheckError(AudioUnitSetProperty(player->outputAU,
                                      kAudioOutputUnitProperty_EnableIO,
                                      kAudioUnitScope_Output,
                                      0,
                                      &flag,
                                      sizeof (flag)), "Set io property failed");

    // Register render callback

    AURenderCallbackStruct output_cb;
    output_cb.inputProc = recording_cb;
    output_cb.inputProcRefCon = player;
    CheckError(AudioUnitSetProperty(player->outputAU, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Global, 0, &output_cb, sizeof (output_cb)), "callback register failed");

    // initialize graph

    CheckError(AUGraphInitialize(player->graph), "graph initialization failed");
}