I find this link that can be useful to configure the multimedia keys on a keyboard: HOWTO_Use_Multimedia_Keys
The main problem is often as those keys don’t retrun any keycode with xev. This Howto explain the whole process to get those keys to work. I added some examples for FVWM.
I really can’t be bothered to edit that wiki section, but note that as your additions (I am assuming you added the FVWM and FVWM-Crystal section) certainly aren’t “portable” since you’re using $[mod0] which is only defined in FVWM-crystal. A much better way is to completely ignore it in lieu of using the output returned from:
xmodmap -pm
I’ll let you make the necessary adjustments.
– Thomas Adam