Shouldn't that be "en mia sandviĉo"?
"en" would mean in. "sur" means on. You could say "On my sandwich" or "in between my sandwich(es)". But I'd say that "in my sandwich" sounds a little odd, like whatever you put on is merged with the bread of your sandwich.
Sur = on
En = in
In English, you spread butter ON the bread. But fillings like meat or cheese you put in it, as in between the slices of bread.
That said, if someone were making you a sandwich they might ask you if want meat ON it. English...
I agree, though I have not heard anyone talk in English about having meat ON a sandwich.
I keep answering all the audio questions in English; shouldn't it say "type in Esperanto" or something like that?