Have you explored Normalizing the clips....Clip Menu/Audio Options/Audio Gain
Please see "Adjust the Input Level of Clips" in the following Adobe document
Are you working with Music Score Soundtracks?
Are you working with 5.1 channel audio?
Generalizing...you should be able to volume change an audio clip that has been trimmed.
This may not have been the best way but I got it to work by sliding the bar down as the clip was playing. Seems to me there should be a way to have the timeline marker at the start of the clip and then move the bar down so when the clip starts it the sound starts at the lowered position, but I couldn't get that to work. (To me, that would have been the easy way to do it, but I'm used to Pinnacle on Windows.) I could also select the clip and adjust the gain which also lowered the volume at the start of the clip.
Thanks for the update and what worked for you. Good job.
Is all your work with volume changes using the Audio Mixer? Have you explored doing volume adjusts at the level of the Timeline audio clip's rubberband?
In Audio Mixer, does it help to set and hold the audio track slider to the wanted volume as you open the Audio Mixer?