The level designer is a sort of mix of a programmer and artist, as they construct the challenges and environments for players to face. They'll often use a set of tools created by programmers but apply their own script to the environment to achieve the desired vision. The challenges that level editors create include various things such as missions or goals for the players to strive for. Often they work with semi completed art work to first get the general layout of the environment they want to create.
These are the people in charge of creating the sounds that will be featured in a game. Any audio is over seen by the sound engineer, this includes composing for background music, sound effects and voice acting. Their job is fairly simple in that they create the sounds the designer wants, as with other departments there will be a head sound engineer who will work closely with the designer to get the best sounds that match the designer's vision. Sometimes Composers are outsourced.
Game testers are the one who play and analyse the game for any faults that might come up during gameplay, they also check that the game meets the desired vision of the designer. This often requires a good knowledge of computer design and competence in analytical skills. Often this process can be expensive so it's often completed at the end stages of the game design process.
With the roles defined it's time to outline the order in which things work. At the top will be the publisher, who are in charge of the producers who oversee the complete project, below them will be the designers, next programmers as they bring together all the aspects of the game and put them together, below the programmers will be the Artists, Level Designers and Sound engineers. Resulting in the whole process looking a little something like this:
Publisher>Producer>Designer>Programmer>Artists, Level Designers, Sound Engineers.
You'll notice how I haven't included the Testers, this is because the tester whilst are a part of the game design process don't contribute to the creative process in the same way. Testers act more like debuggers and quality checkers to make sure the game is proceeding at the desired level of quality.
After this look at the roles in game design companies I've certainly more enlightened about the components and job oppertunities in the industry as well as potentially finding my prefered role should I enter the employment of a geames studio.