SPOILERS FOR ARCANE S2 EP 9:
Throughout S2, all we've heard of Viktor's musical motif has been incomplete, distorted or hidden beneath other overlapping themes.
Viktor's theme only plays fully and purely in the last few minutes of Ep 9, when Jayce puts his hand on Viktor's shoulder in the astral realm.
Viktor's expression falters after he says this, because the resulting pause is leading him to assume the worst—His face falls because he's expecting Jayce to listen and leave him to die.
But no, Jayce reaches for him, and touches him in the same way he has for years, ever since the first night in the ruins of Jayce's apartment. Jayce making that human connection brings Viktor back to his own humanity—it literally brings him back to himself.
And so, right as Jayce makes contact, we hear Viktor's full theme play fully for the first time in S2—and it just so happens to be the most triumphant and beautiful and orchestral rendition of it that we've ever heard in the show.