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.
xset r [keycode]
to turn on auto-repeat for key
xset -r [keycode]
to turn off auto-repeat for key
keycode. Note that the bar means minus, not option.