Everytime there is an update, Windows resets the volume level (in game and for the client).
Is there a way to fix it ?
In your config folder there is a file called persistedsettings.json that is getting recreated on launch. This is a new addition to their file system and I think the settings are in here.
Try moving this file out of the directory and replacing it with a config.ini file, which is used to load in game settings. I haven't had this problem since doing this.
One way i am aware you can do this is by using LSI(League Summoner Information) which was one of the reasons i started using it actually, you can control at which volume LoL stays even after updates.
There might be other ways, but this is the one i use.