I think I've figured out what's going on. I played around with all four versions of OrgMaker (1.34 Japanese, 1.34 English, 2.05 Japanese, 2.05x English) and they all do the same thing.*
In each track, when a note is played with a different volume than the previous note, it actually starts out at the old volume then is quickly changed to the correct volume. Unfortunately some of the drum sounds are very short, so by the time the volume switches to the volume indicated in the score the drum sound is practically over and you don't hear the new volume.
Some examples:
volume-delay.zip
a. In vary-note.org, I hear the notes play at the old volume then quickly change to the new volume.
b. In vary-drum1.org, the Bass01 drum sounds okay.
c. But in vary-drum2.org, the Bass02 drum effectively ends up using the volume value of the previous note.
So a possible workaround is to change the track to a different drum sound.
This "volume memory" for each track is also changed by the example notes OrgMaker plays when you create a new note, click on a track button, click on a piano key, or change instruments in the Instrument Settings. The next note played on that track will start out at the example volume, then quickly switch to the volume from the score. Also when you first open a file, the "volume memory" for every track is set to a large volume value. The first note of each track is played loud, then quickly switches to the volume from the score.
*I think it may be a DirectSound audio acceleration effect and not a bug in OrgMaker itself. On my Windows XP, I notice if I change the
"Hardware acceleration" setting to "None" then the volume is correct at the start of the note, but then the overall timing of notes becomes jittery. I haven't tested the volume effect of all the settings yet, and don't have other computers to test if it sounds different on other versions of Windows...
It's late and I need to go to bed...