Yup, if you made it, and they already released it publicly, you're pretty safe to add parts of it to your portfolio.
There is always an element of risk to anything you do, but in this case as it is limited use of something you built for another project, and it is a part of your career, and the company already released the music for general consumption, that is not a particularly large risk. You are submitting a portfolio of samples of things you have done before, not publicly posting trade secrets from inside the company.
The courts have established a four-prong test to help guide if the infringement is actually a fair use of the product. It is still a judgement call rather than a bright line, but building a demo reel to showcase your work history would almost certainly be immediately dismissed by the courts. While it doesn't have a so-called "bright line" of right and wrong, within the huge gray area it is in the "it's okay" region. The risk is not quite zero, but less risky than other acts like driving a vehicle or visiting a public restroom. Extremely safe, but not perfectly safe.
Exercise some care to ensure that your use is brief. Use enough to showcase what you have done, not so much that it becomes an alternative to the game's soundtrack bonus content.