Managing audio in Storyline

Hello, I'm new to Storyline, and I appreciate your help.

I have a slide with 2 audio tracks, one for narration and one for music. The music starts right away and the narration starts at 6 seconds. I want to music to reduce in volume when the narration starts.

I did this in by double-clicking on the music and selecting the area from 6 seconds onward. I reduced the volume. However, the transition is abrupt and not easily undoable, and appears to be more or less permanent.

I ended up having to mix the narration and music in Adobe Premiere and importing it as a single track. Are there any controls for audio, such as transitions or keyframes for rubber-banding volume which I'm not seeing? Or is it necessary to get the correct audio levels done in another software and import it from there?

Thank you for your help.