Thank you for sharing! The lower volume is because VoiceChat uses a different audio unit which has the characteristic of playing system sounds, but reducing their volume by an undocumented extent (i.e. the volume drop changes from OS version to OS version) because it also does some amount of echo cancellation.
I don’t use it as the default because its behavior isn’t contract behavior (it isn’t described in the docs so there is no absolute promise about what it is supposed to do) and my experience over the years is that Apple does less testing of it than the Remote IO unit, meaning that occasionally an OS version will come out that breaks it briefly. However, I think it’s OK for individual projects to use as long as you test it occasionally to make sure big OS updates don’t affect it.