Yes, I recommend you investigate. A company always has some inherent liability in relation to discriminatory or harassing comments or behavior. The level of liability usually correlates to the nature, severity, and context of the comments, the position of the employee who made them, and what the employer does or does not do about it.
Since you have knowledge of a potential situation, I recommend you investigate the matter and take appropriate disciplinary action if it turns out your anti-harassment policy was violated. As you conduct the investigation, document the discussions you have as well as your findings, and reassure those you interview that their participation will not result in retaliation.
If you need additional guidance on conducting an investigation, please let us know.