An audio designer, sometimes called sound engineer, is a team member responsible for a game's sound effects and music. Audio design is often provided in a freelance capacity rather than as a full time position, but this varies per studio and its requirements.
Audio is one of the most underserved parts of game production, but also one of the richest when done right. Great audio can really help pull a player into a game, but studios often don't think about it until quite late in the production process.