First of all, I am beginner on this board. I want to take a music from USB, convert it, and give it to analog output. There is Linux Debian on the board. Could you please explain to me how to do that from scratch?
As you are looking for a very broad walk through of helping with your project you might have more luck looking on the Qualcomm Developer site where they have a number of projects
For example, this one has been done with the goal of demonstrating the audio functionality of the DragonBoard 410c:
Hope that helps.