Voice is the combined effort of numerous muscles.

So, why should it surprise you that it fluctuates like that in an illness whose hallmark is fluctuating muscle weakness?

If the respiratory muscles are the prominently weak muscles you will sound breathless (=anxious) when you talk, if the laryngeal muscles are effected more it will change the pitch of your voice, if it your palate it will become nasal etc etc.

For me it was very embarrassing that my occupational physician has a name I find hard to pronounce
With time I have found the solution for that-I call him by his first name.