No you do not - at least not technically! I had to look into this at work. I looked at WebEx, Teams, and Zoom - all three let auto-transcribe just roll. I think on one of them it gives a quick popup saying it's transcribing but no consent required. Contrast with audio/video recording where Zoom lets you either consent or leave the meeting. I asked our legal counsel and they said it's kind of iffy whether live auto-transcription counts as recording - didn't seem like a settled matter.