Hello Jonas.
You can leave the audio line disconnected, it's an optional thing if your camera has a mic and an audio output then you can feed the audio into the vtx and get that mic noise into a headset on the ground. Mr. Steele is the only pilot I've ever see do this on YouTube and I've never done it myself, I could see the value in hearing if something is going wrong but if things are flying strange I usually just fly the quad nearish to myself to hear it first hand or land and check it out on the ground.
The FC takes the video in and has video out so it can add on top of the camera video to give you the on screen display (overlay with info about battery voltage or altitude or whatever you customize in the betaflight configurator OSD tab).
If not using OSD you can just go straight from the camera video out into the vtx video in and power both up and you'd have a video feed being broadcast.
There is one more thing to point out, if (iff) the thing you're referring to is "SmartAudio" that isn't actually audio despite the name. "SmartAudio" is used to send control signals from the FC out to the VTX, this way you can use your transmitter (already connected to the FC via the control receiver) to send out control signals to change the VTX settings (primarily to adjust power or channel selection). Typically I just go with VTX that has a button on it and clear LED display of what I'm setting to pick channel and power output (AKK ones in particular have been good to me) and I don't mess with SmartAudio... I did hook it up on my hexachopper but haven't used that one much due to other issues (signals to ESCs being wonky, it has flown but is scary and does some weird things so I need to work on it more on the ground).
