Hi,
What do you mean by non-default device?

Typically the mic or mixer can easily be exposed, even bluetooth mic (although naturally very laggy).

I have not worked with QtMultimedia in the past, as it hasn't proved flexible enough for my needs when I've looked at it. GStreamer is available on the device and may well be an easier route to go for development. It's not a natural fit with Qt but Gtk/GStreamer is fine with C++.