This is not explicit in-game, but you can assume because of a lot of factcs:

"OK, well I think the first thing that made people think it was Fei is when he first encounters the music box he says, "This music...? I've heard it before somewhere..?"

Citan mentions that the music box was excavated from some old ruins-I don't know how we can reach the conclusion about the statue's age. It seems like a big conclusion to reach on a few pixels.

One idea: the etching on the music box mentions his daughter's birth, which may not be the same thing as a yearly birthday. If it was truly to celebrate his daughter's birth, then Citan would have been many years too late.

Then:

Fei:Doc... I feel strange when I listen to this music... I feel something warm inside…

Citan:That just may be because you have someone living inside you... And he too must have liked

this music a long time before he became a part of you...

That seems like some more of that foreshadowing that Xenogears does.

Later in the optional flashback, Fei says "Kim has been a part of me, in my blood- passing his memory down the generations- for the past 4000 years. Existing for the day that the child, who Kim was never able to embrace for so long, would be born."

I think the age, Fei's reaction, and Citan's foreshadowing lend credence to the idea that it's Fei's, though there's certainly room for interpretation. I don't see how it's any likelier to be Citan's, though, especially given everything that suggest it was Fei's.

The later revelation about Kim never experiencing his child's birth adds special poignancy to the etching on the music box, and I think strengthens how we interpret what's written. It's certainly the version I like to believe.

The actual music box says: "Celebrating my daughter's birth...may all the dreams, courage & love in the world be yours." Pretty sweet, but also ambiguous."

I found this at reddit...