Unreal Engine 5.1 with Agora SDK 4.2.1: Audio Input is not transmitted when using Pixel Streaming.

I’m working with Unreal Engine 5.1 and Agora SDK 4.2.1 on Windows 10. I’ve got two Streamer instances running on different PCs in different local networks, with a phone connected to each streamer. I used the sample app from the AgoraUnrealSample GitHub repository: GitHub - AgoraIO-Community/AgoraUnrealSample at AgoraBlog. I’ve installed the Pixel Streaming plugin and placed the Pixel Streaming Audio Component on the level.

The users can communicate via audio while they are at their PCs, without using Pixel Streaming. However, when they speak into their phone microphones, the other party on a different streamer cannot hear anything. The local echo test works fine on both ends, meaning if they speak into their phone microphone, they can hear their own voice.

What could be causing this issue, and how can I resolve it?