Assistant sings(?) unexpectedly

I've had a weird interaction while testing my assistant. At the end of the call it unexpectedly sings(!) for a couple of seconds. It's not logged in the call transcript, but it can clearly be heard in the recording.
What could be the reason?

Model: gpt-4o
CallId: 232cef00-c77a-42ee-91f9-317df0c6009b,
it's in the last 10 seconds of the call, I've attached them
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