Translation:They do not eat sandwiches in the bar.
Its not in the hearing. There are no sandwiches (plural). The 'de' is used in the negative, but the plural is implied, because "there are no more" OF THEM (plural). In the singular it is the same, because "He doesn't eat "sandwiches" at the bar, NOT "He doesn;t eat sandwich at the bar."
My grammar book agrees: 'The partitive article is replaced by de or d' in negative expressions (with minor exceptions).' Example: Je n'ai pas de verres. The de is followed by plurals, as Jonc967 says. So once (or if) we hear n' ... pas de, we are supposed to know that sandwiches is plural.
In a negative you do need to change un and une to de, but you don't change the noun from singular to plural.
- Il a un sandwich.
- Il n'a pas de sandwich.
At any rate, I find many samples of singular negative phrases like those here: