I had several Steam multiplayer projects in 4.14 that were using voice chat and worked correctly. After upgrading to 4.15, voice chat only works in the lobby and cuts out after server travel to a new map.
Repro steps:
Add an Execute Console Command node.
Set it to “servertravel yourmappath”
Call that.
Voice chat stops working.
There was another thread here → https://answers.unrealengine.com/questions/580964/steam-voice-chat-stops-working-after-server-travel.html
Unfortunately, it got “Flinted” then abandoned.