From Forum Nokia Wiki

External volume buttons

From S60 3rd Edition FP 2 onwards, external volume buttons can be used to change the volume level of MMAPI players in mobile devices that have the volume buttons present.

The buttons work in this way when a MIDlet that uses MMAPI is on foreground. They change the volume level for MMAPI Players globally, that is, the volume level of every Player in every MIDlet is altered.

A Nokia proprietary event is used to notify the MIDlet when volume is changed with External volume keys. The notification is made via PlayerListener.playerUpdate(Evendata) callback with the event com.nokia.external.volume.Event.

Code Sample

The event can be decoded to know the current global volume level as shown below:

// Effective volume considering both midlet volume and global volume
int actualVolume = 0;
int globalVolume = 0;
 
public void playerUpdate(Player p, String event, Object eventData) { 
  // get the current midlet volume level
  int midletVolume = vc.getLevel();
 
  if(event.equals("com.nokia.external.volume.Event")) {
     // get the current global volume level
     globalVolume = Integer.parseInt(eventData.toString());
  }
 
  // This event is generated when midlet volume level is changed
  if(event.equals("volumeChanged")) {
    midletVolume = vc.getLevel();
  }
 
  actualVolume = (globalVolume/100) * midletVolume; 
}

Reference

http://library.forum.nokia.com/index.jsp?topic=/Design_and_User_Experience_Library/GUID-CB5D4F7A-CA69-49E6-839D-2F7E30641498.html