-Shameless self promotion warning-
I've patched/updated an addon by Jan/mooNylo's which sounds like it's what you're perhaps after (but currently without sound).
ExecuteNow - Update - it shows an alert on screen (moveable via settings) which allows for a custom execute %, time out, target's minimum total HP (to bother with the alert), and size/opacity tweaks.
Jan/mooNylo's original neglected non-class based executes (such as the 2 Handed weapon's "Reverse Slash") and lacked the Warden and Necromancer class.
Currently it shows the prompt once the target's health drops to the defined % even when your Ult is not ready so in its current state the update may be a little spammy - I do plan on fixing this as soon as time permits & may add a sound option once it's not as "spammy" (as Baertram noted, we're limited to ingame sounds currently)