Is there an auto ducking feature I'm missing?

Legacy FCP had auto-ducking, and so do all other NLEs, and they've all worked great and have been accurate for some time. It's old had to code that. If you want to auto-duck in FCPX you can't. But there is a utility app that does this for you, and will round-trip with FCPX. I use it all the time, saves so much time, and is super accurate.

FCPX Autoduck

http://ulti.media/fcpxautoduck/

Personally, I'm still astounded that Apple leaves such a standard and super handy and super time-saving tool out of FCPX.

Is this what you want?

Press the R key to turn the cursor into the Range Tool and drag across the area where you want the audio lowering.

Then drag the volume line down and you will notice there is a gentle slope which prevents an abrupt change.

There are also keyframe markers which can be dragged to make the volume change shorter or longer to your satisfaction.

If you really must have automatic audio-ducking you need an editing app like the free version of DaVinci Resolve.

This app can do many things that FCP can't, including auto-ducking which is described in this 6 minute video.

https://www.youtube.com/watch?v=8wfzepEjZpM

Essentially you put your music below the VO track and use a sidechain compressor in the Fairlight panel to lower the volume every time the voice kicks in.

The process is much quicker and easier than would appear from a first glance at the video and it only has to be done once to affect the whole video.

You can download the free DaVinci Resolve from here . . .

https://www.blackmagicdesign.com/uk/products/davinciresolve

The paid version (Studio) costs £245 but the extra features would not be things you need or could use.

Here's a very short tutorial about ducking audio (quickly) in FCPX:

https://youtu.be/rjYkF7RmTHg

So just trying to figure out if this is just one of those things I have to do manually every time.

What kind of volume adjustment are you using specifically? What task do you find most tedious?

I think adjusting the volume of narration and BGM is very important. I like to make detailed adjustments until I'm satisfied, so I've never felt like relying on the automatic features of an app.

The tutorial shown by fox_m explains how to cut BGM →select a clip →lower the volume →soften the volume difference with a transition.

You can soften the difference by lowering the volume without cutting the BGM. Select the range you want to lower the volume with the I and O keys →Lower the volume bar while pressing control+(-minus) or ⌘.

You should now have four keyframes inserted between I and O.

No ducking here . . . 🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆🦆

👍😆

I knew you would improve on my offering! We'll be driving everyone quackers.