There currently is a way to smoothly change from one audio level to another, but it involves edits and fades. Having a filter that starts at level A and gradually ramps to Level B with a specified duration will be welcome. Adding curves (e.g., change quickly to change slowly) will help to make the change more gradual.
You need an audio editor for this kind of work. NLE’s concentrate on Video and simple gain.
For what you are asking try Audacity’s envelope tool.
I use Audacity regularly. Thanks for the reminder on envelope shaping.
What I haven’t resolved is transferring what amounts to key frames or in/out points from Shotcut along with the audio track(s). And sometimes it’s just “gotta see it as I go”.
For example, in my stock intro clip for my Alps videos, I backed into finding that the cut from the first title to the second title just happened to drop on the first beat in a measure. Armed with that, I timed the subsequent cuts for the subsequent first beats. Call it serendipity. Anyway, the result is a set of titles with a bit of punch to it.
If I’m working in Audacity, I may get the audio sounding good but not quite reflecting with what’s on the screen. Which is why I suggested the sound ramp filter. Change - play back - change - play back… until it looks and sounds right.
Until Shotcut devs include keyframes, we’ve got to use workarounds.
Some of my workarounds impress even me. LOL