Hi Estapley,
welcome to the forum!
Multiple volume changes of a channel can be made in the "Step Edit" list:
Go to "Function > Menu 2 > Song Creator > Tab 1 - 16" (Refer also to the S975/775 Reference Manual, starting with page 58.)
First press the "Filter" button and, if needed, press "CC" button, and make sure that the "Control Change" events are NOT filtered out (page 74).
Press "F" button to select the desired MIDI channel. At the beginning of the song (in the Event List of the "1 - 16" tab) you should see a CC #07 (Channel Volume) event.
To change the Volume during the song you can insert additional CC #7 events on the desired time positions. But be aware: If using more than one CC #7 event (= absolute Volume), it is not easy to change the global channel volume later, if needed.
Therefore, I recommend to use CC #11 events (Expression) to set different (relative) volumes during the Song.
Note: If you use CC #11 (Expression) events, you can only LOWER the volume. If you insert an event "CC #11, value 127" you will still have the full volume (set by the CC #7 event at the beginning of the song). Then you can decrease the CC #11 value to lower the volume as desired. (If you need a HIGHER volume during the song, you have to increase the CC #7 event at the beginning.)
Hope this helps!
Best regards,
Chris