I may be wrong but I believe the law on recording others’ conversations is that you can’t do it without their knowledge, but that doesn’t require their permission. Permission is effectively granted if the person being recorded continues speaking after being told they are being recorded.
Turns out I was kind of wrong, but every state has their own version of laws on recording voices, which you can find HERE. You need everyone being recorded’s permission in some states, and one person’s permission, if they are participating in the conversation, in others.
However, I think you could still argue that someone continuing to speak, after you have told them you are recording, may be considered de facto permission.