The Original Game Installed with many different Languages incorporated into the Game.
This made the Language Menu Appear in Game.
The Very 1st Update after the Game's Launch had broken the Language Support in Game.
If you Install a 2nd Language you can get the Language Menu back in the Game, but I don't know if it will fix the Audio.
You are Welcome to try it out. In the Past I used to send a copy of the Audio Files for the Game to other Gamer's.
Or it was that I used to try & find a way to send them the Files, but they were too large in size, & I just was never able to do it. I forgot, it's been such a long time since I played it.
I dunno if I have a copy of the Audio Files anymore, though.
The reason for the Audio breaking is likely this?
English = Audio & Subs
Any Other Language = Subs Only Now
Without the 2nd, 3rd, & such Audio Options, the Audio Menu has disappeared, & won't return without them being added back to the Game.
It does not appear that Ubisoft has a copy of the Audio Files anymore, either. I dunno what happened.
Again? You are welcome to try & Install a 2nd Language, & see if it fixes the issue.
If it does not fix the issue, then we might be out of luck here.
The 2nd Language is not for you to switch Languages, it's only purpose is to bring back the Audio Menu,
& that is it. From there you would Select your Audio Language, & hope that it works.
Good Luck!
PS: This requires a Full Game Re-Install I think, I might be wrong.