Scared of drive thru at work?

So I have auditory processing issues (not determined by doctors though) where sometimes as people talk, their words mush together and it doesn’t sound like they’re saying words at all. It’s worse via phones or radios, etc. Yesterday, I was training and they had me wear a headset to hear the order in advance so I could start making it before they pulled up. I barely could understand or hear the customer even at full volume (especially with machines going off that crushes ice). 

They have everyone doing everything. So mainly there’s one girl at drive thru but if someone else is busy making a sandwich or they’re with a customer inside, the other person who’s available would do drive thru for them. 

How do I ask if I can only do making drinks/food and doing the inside register? I don’t have a doctors note about the hearing thing and don’t have the money to get it checked out. 

But it’s terrifying me thinking that tomorrow I might be trained on drive thru and completely mess up. 

They’re chill there, but they cross train on everything so I’m unsure if they’ll make an exception for one person who can’t even show a doctors note. 

I just don’t know how to bring it up or ask for an exception without sounding weird or like I’m trying to be lazy/skip out for no reason.