Control volume using firmware?

I use a Radio Shack and/or Koss “volume cable” between my player and headphones, and it works great–making it the easiest way to change volume on-the-go.  And, it adds some extra length to your phones’ cable, if that’s desired. The Radio Shack version is around $10; the Koss around $7.50 on Amazon.com.  Shure also manufactures a version.