Comment 1 for bug 595923

Thank you for reporting the bug. I have tried it on a HP Probook 4710s, but the volume down shortcut with Fn is working as expected. So it's not something generally HP Probook related.
About testing laptop keys you can find some info here: https://wiki.ubuntu.com/LaptopTesting/Keycodes
I have tested the keycodes for volume up and down with the xev command, but the output for these did not differ. A method using the showkey in another TTY is also described there, that gave me the keycodes 114 and 115 for volume changes. Maybe you could test this too.