Secret agent that also sings opera and travels through windows

I read this story about 5 years ago, but it was not new at the time. It was probably written in the 90s to early 2000s.

It was part of a series, I think referred to as the author's "Hundred World" saga. It was not the first book of the series.

I do not remember the cover.

The only details to the story I remember are these:

The main character is some sort of secret agent. He works for an organization that polices travel by these devices that allow you to step from one place to another.

At the beginning of the story, he's preparing to sing operatically for a sold-out audience, because his public identity is a world(s)-renowned singer. He prepares by drinking a special blend of tea and honey to soothe his throat.

The appearance was actually a bait tactic to lure out assassins. They appear (using the instant-travel devices, I think?). I can't remember how this scene ends.

I remember that this character's "boss" is someone they call a nickname like Chief, and Chief's history is classified. But, eventually you find out that the window devices that let you travel from place-to-place originally came from another world (maybe dimension?), and that the Chief came from this world (but was a child at the time). I remember this was supposed to be a big reveal, but since I didn't read the previous novel about Chief's character it didn't mean much to me.

I don't remember much else, aside from this book or another in the series taking place on a different planet where the government was either very religious or very picky about the language you spoke.

I may still own this book, but it's likely inaccessible in storage if it wasn't already donated.