So the issue has still not been solved, but here is a little workaround for CS:GO.
According to this link:
https://wiki.alliedmods.net/CSGO_Quirks#Workarounds
You can add a * (asterisk) at the beginning of a path to an mp3. Please note that there is no workaround for wav.
This means you'll have to convert the wav files to mp3 and change the config file <moddir>/addons/sourcemod/configs/time-left-actions/default.conf in order to get it to work.
Please test and let me know if this works.