If the game does not run at 100% speed, audio will go slower and stutter. That's not a bug, it's just how GameCube audio works.
The performance of PC games has no correlation with performance of games in Dolphin since Dolphin is a lot more CPU intensive than most PC games.
To make the game a bit faster, use EFB to Texture in Graphics Config > Hacks. This might help with your performance issues.
Disabling shadows rendering isn't possible if the game does not provide the option in the first place. It also wouldn't help with performance at all. I don't know what gave you this idea, but it's wrong.
The performance of PC games has no correlation with performance of games in Dolphin since Dolphin is a lot more CPU intensive than most PC games.
To make the game a bit faster, use EFB to Texture in Graphics Config > Hacks. This might help with your performance issues.
Disabling shadows rendering isn't possible if the game does not provide the option in the first place. It also wouldn't help with performance at all. I don't know what gave you this idea, but it's wrong.