It can mean either! 1. The person who can't be heard is at the back. 2. The person who can't hear is at the back. You'll have to figure it out from context.

It could be both making those statements. The listener (asker) if they are in back of room and cannot hear the speaker very well. The speaker may say statements if they cannot hear someone in back of room and need them to speak louder to be heard.

Usually we say "I can't hear you in the back"... And it refers the person who's speaking. Generally, it means that somebody in the back of a classroom or auditorium is speaking in a voice so low that it cannot be heard clearly by the professor/speaker, etc.

The listener is at the back. Think of a room full of people. Someone is giving a speech and a person in the back of the room can't hear. So they shout out "Speak up please! I can't hear you."