I love fast auto-repeat but I don’t want it happen on the XF86AudioMute keys. What surprised me was that I found xset provides the function of configuring auto-repeat on/off for a specific key.

First, use xev to get the keycode.

Second, run

xset r [keycode]

to turn on auto-repeat for key keycode.

xset -r [keycode]

to turn off auto-repeat for key keycode. Note that the bar means minus, not option.