Is there any "chisaran" (is there something "lacking") in one's fulfillment of the Mitzvah of Talmud Torah if they listen to soft, slow paced, instrumental music (no vocals or lyrics) while learning in order to focus better or drown out background noise?
The best way to learn Torah is aloud. That means that we should be listening to our learning.
I wonder whether you can learn aloud if there is background music. In my experience background music is useful if you are studying quietly.
Whatever is best for you. It is probably best to learn aloud but if not, start with music. There is no chisaron (loss) of the amount of reward or quality. If you learn best with music, go for it. Eventually, try to build up to a level where you can shteig and not need the music. I recommend asking your rebbi or rav though, each case is different.