First, I don't know much about these decoders except they can be a PITA.
I would first check to make sure that the wheels and track were very clean. If that doesn't help I would try a factory reset on the decoder. These are just some simple things that can be done without rechecking all the connections.
Lastly, since these decoders aren't known for good quality, and
do not support read back of the CV's, all you can do here is just set the CV's individually, and see how this change affected the locomotive. IOW, it might be best to program in OPS mode, which is programming on the main. You can go here, and see exactly which CV's you may need to change;
http://www.jmri.org/xml/XSLT/pages/MRC_Athearn_MP15.xml.html
The link is a listing from JMRI, specific to the MRC Athearn MP-15 decoder, of the CV's and what they do, giving you a choice of what may need changing. I would print the page to have it in front of you as you change the CV's.
Given the low quality of these decoders, if none of the above helps, I would remove it, then install a regular decoder until I could afford a quality sound unit like Tsunami, QSI, or Loksound, (ESU).