Hi Dennis,
This adjustment is very well hidden

1. Click on the yellow "S" for "Soloist" button and then select "Open Soloist Dialogue ..."

2. Select the "Edit that I've marked in the image below.

3. Now select the Realtrack (#1) and then click on "More" (#2) as shown below.

4. Lastly, add a value in the boxes highlighted below. A positive value will increase volume and negative value will reduce it. I'd start with either -3 or +3. Because these values are decibels, it is unlikely that you'll need much more variation than this.

I'm not sure if this adjustment is global and affects all RTs or if it's for individual RTs (I suspect the latter). It would be easy enough to work out through trial and error, though.
All the best,
Noel