Thanks @johnPayload @pwkmikekelly for the extra details. I’ve been able to reproduce the issue - it seems to be due to how UE5 loads plugins when opening projects.
Essentially, an FMOD system is spun up by UE in the initial loading screen when you open a project, and then once more when the editor opens. For some reason, despite the initial FMOD system being released, the port is still considered as in use, meaning that the second FMOD system in the editor cannot use the port. The development team is currently looking into the issue in more detail.
Your proposed workaround to suppress the warning shouldn’t cause any problems. Unfortunately, Editor Live Update will not work unless you change the port every time you open your project, and I cannot provide any other workaround. However, regular Live Update (i.e. while playing the game in-editor) should still function fine.