You don't say which chips you have, but from the current Massoth configuration manual (Firmware v 4.0) (covering XLS, XLS-M1, XLS-Onboard, LS, S and S-PluG):
CV200 - Valid values are 1...63 or 255 - 1 is low, 63 is loud and 255 uses the value from volume control inputs on the decoder.
So I would try setting cv 200 to 63, which should give the loco it's maximum value if it has a Massoth Sound decoder.