The following link provided the great idea to using the multimedia keyboard keys in your application but this article is for c++, you have to find out by yourself to how should to use those functions.
EDIT :
Based on these articles, I implemented some of media function key which lets you to control volume from your application.
private const int APPCOMMAND_VOLUME_MUTE = 0x80000;
private const int APPCOMMAND_VOLUME_UP = 0xA0000;
private const int APPCOMMAND_VOLUME_DOWN = 0x90000;
private const int WM_APPCOMMAND = 0x319;
private const int APPCOMMAND_MEDIA_PLAY_PAUSE = 0xE0000;
[DllImport("user32.dll")]
public static extern IntPtr SendMessageW(IntPtr hWnd, int Msg,
IntPtr wParam, IntPtr lParam);
public Form1()
{
InitializeComponent();
}
private void btnPlayPause_Click(object sender, EventArgs e)
{
SendMessageW(this.Handle, WM_APPCOMMAND, this.Handle,
(IntPtr)APPCOMMAND_MEDIA_PLAY_PAUSE);
}
private void btnMute_Click(object sender, EventArgs e)
{
SendMessageW(this.Handle, WM_APPCOMMAND, this.Handle,
(IntPtr)APPCOMMAND_VOLUME_MUTE);
}
private void btnDecVol_Click(object sender, EventArgs e)
{
SendMessageW(this.Handle, WM_APPCOMMAND, this.Handle,
(IntPtr)APPCOMMAND_VOLUME_DOWN);
}
private void btnIncVol_Click(object sender, EventArgs e)
{
SendMessageW(this.Handle, WM_APPCOMMAND, this.Handle,
(IntPtr)APPCOMMAND_VOLUME_UP);
}
Just copy and use wherever you want.
Low-Level Keyboard Hook in C#
Using Multimedia Keyboard Keys in Your Own Program