If someone clones my voice and calls my bookkeeper asking for a transfer, what actually stops it?

The uncomfortable truth is that recognising your voice is no longer proof it is you. A short clip from a social video or a voicemail greeting is enough for cheap tools to produce a convincing imitation, including your way of talking.

What stops it is a procedure that does not depend on recognition. The simplest version: your bookkeeper never moves money because of an inbound call, voice note or text. They hang up and call you back on the number they already have saved. If it was really you, you lose thirty seconds.

A code word works as a second layer — something agreed in person, never written in email or chat, that you say when a request is genuine. Change it if anyone leaves.

Tell your bookkeeper explicitly that they will never be in trouble for refusing an "urgent" request from you until it is verified. Scams like this work by making the careful person feel they are the one slowing things down.