[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, uint wParam, uint lParam);

const uint WM_APPCOMMAND = 0x319;
const uint APPCOMMAND_VOLUME_UP = 0x0a;
const uint APPCOMMAND_VOLUME_DOWN = 0x09;
const uint APPCOMMAND_VOLUME_MUTE = 0x08;

SendMessage(this.Handle, WM_APPCOMMAND, 0x30292, APPCOMMAND_VOLUME_UP * 0x10000);

SendMessage(this.Handle, WM_APPCOMMAND, 0x30292, APPCOMMAND_VOLUME_DOWN * 0x10000);

SendMessage(this.Handle, WM_APPCOMMAND, 0x200eb0, APPCOMMAND_VOLUME_MUTE * 0x10000);