Sorry: no.
Since the introduction of AirPlay 2 Apple decided to have audio sources with different priorities. This means that a source streaming via AirPlay has to be seen as of a high priority enough to actually be send to an AirPlay target.
For Sigmondo this means that only sound from a video is streamed if the video plays in fullscreen and the controls (play, pause etc) are enabled. Being a self-sufficient signage app this is not useful for Sigmondo. It even lowers the purpose of Sigmondo.
We created a small player app with these functions to find a way to make this work again, but there are too many disadvantages. So we decided that the ‘sending audio from Sigmondo via AirPlay’ feature will be nothing we can provide at the moment. Sound will play on the TV or on external soundbars which are physically connected, but not via AirPlay.