Can't make media keys work on i3 window manager

Make sure you have the correct keysym in the first place – for ⏸ it should be XF86AudioPause, for ▶ / ⏸ it might be XF86AudioPlayPause, or it might even be something completely different. (XF86AudioStop is the ■ "Stop" button.)

The keysyms for volume controls should be XF86AudioRaiseVolume, XF86AudioLowerVolume, and XF86AudioMute.

For checking X11 keysyms of a key there's xev and xbindkeys --key. (Note that they often won't see keypresses for already globally-bound keys, so don't use them in KDE.)

(Low-level evdev keycodes can be obtained with evemu-record or libinput debug-events; they're not the correct kind of code, but are useful to see whether the key is recognized at all by the kernel.)