But that means the voices get out of sync during cinematics if your fps is too low

No, the game uses frame delta times rather than a set tick rate per second.

(In general it's how most games already work; it's just that it's normally convenient to set it at a fixed rate to make some things like physics more deterministic, and it gives a nice target performance/budget).

Not if the cinematics are synced to a global timer instead of the frame rate. Then when the frame rate is too low the game skips some frames to keep up.