As of right now, the only native software audio mixer for Windows is Voiceemeter Banana. Using VB allows for streamers to listen to music while streaming and not having that song's audio be broadcasted, or natively change the volume and stuff of games and other sources and stuff interactively. Essentially, a native audio mixer would allow you to record/stream and control specific audio sources directly through OBS.
While there are software mixers available for PC, like Voiceemeter Banana, setting them up is quite a chore and quite confusing too. In fact, there's even been multiple guides about setting this up, so you know it's a commonly used feature and wanted by the community! Integrating this functionality directly into Streamlabs would be awesome. It would also work super well with the Streamlab's minimalist design.