On a listen server, any voice chat sound that should be routed through a UVOIPTalker component doesn't appear to do so. The sound is audible regardless of where the host's player is located in relation to the other players (the sound is located at the origin) despite the UVOIPTalker component appearing to be correctly registered and configured. This doesn't occur if all the instances are PIE, only when one or all are standalone.
See attached repro project. (In order to test VOIP in the project, the instances should be launched as standalone. You can then press H to host a session and J to find and join a session.)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-131068 in the post.