var input1 = new WaveFileReader(файл1.wav");
var input2 = new WaveFileReader(файл2.wav");
var waveProvider = new MultiplexingWaveProvider(new IWaveProvider[] { input1, input2 }, 2);
waveProvider.ConnectInputToOutput(0, 0);
waveProvider.ConnectInputToOutput(1, 1);
WaveOut Device = new WaveOut();
Device.Init(waveProvider);
Device.Play();