"Do you want to play music now?"
Translation:¿Quieres tocar música ahora?
If specific music or music in the most general sense was implied by the English, then you would include the article la. Otherwise, if it's not in the English, then it's not going to be in the Spanish.
You will encounter English sentences where a definite article is implied, but not stated. You will need to include the article in a correct translation to Spanish. This will make it appear as if Spanish doesn't follow any consistent rules with regard to definite articles. Or, it may make you think Spanish uses definite articles much more often than English. My impression, however, is that Spanish is just more consistent in its use/expectation of definite articles than English. Actually, it's probably more accurate to say that Spanish as practiced on Duo is more consistent. I believe that native Spanish speakers often drop the definite article when it should be used.