Mande

When said with rising intonation, ¿Mande? is a common way in Mexico of asking someone to repeat something you didn't hear the first time.

— Mañana ... (inaudible).
— ¿Mande?
— Que mañana abrimos temprano.

Although mande is a subjunctive form of the verb mandar, give orders, its form here is invariable. You don't change the ending even if you have a tú relationship with the speaker or if you're speaking to a group. Apparently the roots of mande lie in Mexico's colonial past, when indigenous workers fared little better than slaves. For this reason, some speakers find the term distasteful and reeking of servilitude. A standard alternative is ¿Cómo?

A Communication Strategy

Asking speakers to repeat what they said is a communicative necessity for learners. However, try not to rely too much on mande, since it doesn't communicate which part of the utterance eluded you. If someone says Voy a ir a ... (inaudible) but you didn't hear or understand the last part, you can respond with ¿A dónde (vas a ir)? You don't need to make them repeat the whole thing. Also, this helps them focus in their reply on the part you actually need repeated. It's a win-win.

Que When Repeating

By the way, did you notice in the first example of this article that the clarification is prefaced with que? That's normal. And had the inaudible part contained an imperative, the clarification would go in the subjunctive.

— Hazlo ya. Do it.
— ¿Mande? I didn't hear you. Could you repeat that?
— Que lo hagas. I said to do it.

Prefacing a Favor

Mande, with falling intonation, is also a standard way of telling someone who has gotten your attention that they may proceed with whatever it is they are going to say. If someone prefaces the asking of a favor by saying Oye, te quería pedir un favorsote or calls out your name from the other room, you can respond with Mande, meaning, go ahead, I'm listening, tell me. In English we'd probably say something like What? with falling intonation, for What is it? Dime (or Dígame) also work for this.