Some people on the LOTRO music forum have mentioned that the instrument volume in Maestro and the ABC Player is 'off' for some instruments compared to what it would be in game.
This is pretty understandable since no one but Turbine personnel have access to the actual LOTRO code that does the sound rendering, so anything that Maestro does is an approximation of what the ABC will actually sound like in game.
Bruzo, who is the author of the BRuTe midi->ABC transcriber, has created a alternate instrument sounds file, which can be used with Maestro and the ABC Player, that he feels makes the volume sound closer to what it would be in-game.
To use Bruzo's sound file, go to the place where Maestro is installed (this is C:\Program Files (x86)\Maestro if you install Maestro in the default location), and replace the LotroInstruments.sf2 file that you find in that folder with Bruzo's replacement file of the same name, which I have put on Dropbox at this location. You should first rename the original version of LotroInstruments.sf2 to something else, in case you later want to restore it for later for some reason. So if you do this correctly, the version of LotroInstruments.sf2 that you have in your Maestro folder should have a file size of 116,287 KB, and a timestamp of July 4, 2015.
I have done this, and it seems to work ok. I have NOT tested it much yet, so I not really saying that everyone who uses Maestro and ABC Player "should" make this change .. I am just letting people know that an alternate instrument sound file is available. I personally am using Bruzo's file right now on my own system, and if I form a clearer opinion about it as I use it, I will update or add to this post.
BTW, here is the thread in the LOTRO music that discusses Maestro and the ABC Player in general, and towards the very end of it (starting with post number #179 or so) is a discussion of this issue, for those that would like to read more about it themselves.