The sounds are stored in a global array ingame, called SOUNDS.
The synergy ready sound is stored in this filename SOUNDS["ABILITY_SYNERGY_READY"].
You can exchange this sound or set it to ["NONE"] if you don't want any sound.
Try this script in your chat after you logged in:
Code:
/script SOUNDS["ABILITY_SYNERGY_READY"] = SOUNDS["NONE"]
If this works you only need to copy&paste it to the chat after each login (maybe after a reloadui as well, I don't know).