Lowering soundtrack for 30 seconds

[brian]

Add the “Gain/Volume” filter and keyframe the level.

There is an example here:

Fade out audio but only to a certain level

To do this use the Gain/Volume Audio filter. Keyframe the volume decline. (Click on the timer icon to the right of the slider bar.) [shotcut_2020-08-07_09-10-14] You can chose different keyframe types by right clicking on each keyframe node. [shotcut_2020-08-07_09-14-02]

Also demonstrated in this video:

Shotcut Lesson 10 - Keyframes