FYI, if you happen to be on Linux and using PipeWire, you can work around this limitation in any VoIP software that does not yet support sharing desktop audio!
There's a piece of software called qpwgraph (Qt PipeWire Graph) that let's you re-wire the input and output between applications. With this you can wire the audio from your game to movim's input so it will be mixed with your mic's input!
(I'll let you figure out the other fun shenanigans you can pull with this)