waveOutGetVolume()总是返回4294967295(最大值)

请帮忙，为什么在系统音量发生变化的情况下，函数总是返回最大号码4294967295 ?我做错了什么?

我的系统是Windows XP SP3 x86。

class Program
{
    [DllImport("winmm.dll")]
    private static extern int waveOutGetVolume(IntPtr hwo, out uint dwVolume);
    static void Main(string[] args)
    {
        uint currVol;
        int result = waveOutGetVolume(IntPtr.Zero, out currVol);
        if (result != 0)
        {
            Console.WriteLine("Some error occured...");
            Console.ReadKey();
        }
        Console.WriteLine("currVol = {0}", currVol); //always = 4294967295
        ushort left_channel_volume = (ushort)(currVol & 0xffff);
        Console.WriteLine("left_channel_volume = {0}", left_channel_volume); //always = 65535
        Console.ReadKey();
    }
}