Two-way audio enables you to receive and transmit audio through the IP camera. Reolink cameras that support Two-way audio have a built-in speaker and microphone, you could talk through the camera via your Reolink App/Client when you are far away from home. 

Applies to: E1, E1 pro, E1 Zoom, E1 Outdoor Series, Reolink Lumus, RLC-511WA, RLC-523WA, RLC-840WA, RLC-843WA, RLC-540A, RLC-811A, RLC-812A, RLC-823 Series, RLC-824A, RLC-81MA, RLC-81PA, RLC-830A, RLC-833A, RLC-843A, RLC-840A, RLC-1240A, RLC-1212A, RLC-1224A, B1200, D1200, FE-P, FE-W, CX410, CX410W, CX810, Reolink Duo series, Reolink TrackMix series, Reolink Video Doorbell series and all Reolink battery-powered cameras.

You could follow the articles below to enable Two-way Audio for your Reolink Cameras:

Enable Two-Way Audio via Reolink App

Enable Two-way Audio via Reolink Client (New Client)

Note: If you connect the cameras (except for battery-powered cameras) that support two-way audio to the NVR, the Two-way audio function can not be used between the camera and NVR since Reolink NVR does not have a microphone and doesn't support the two-way audio feature. But RLN36 NVR supports the two-way audio feature, so you can connect a microphone and speaker to it or use the Reolink App/Client to use the two-way audio feature for the RLN36 NVR.