audio conversion

There's this guy on youtube called macroblank that uploads these royalty free tracks that's basically just lounge music. I'm going to be away from anywhere with internet and I've downloaded the albums but all of the files are mp4 or webms. I know how to convert them to mp3s but when I do that...

I was recently gifted a basic 5.1 surroundsound system, an Ultimea Poseidon D50. This has limited audio input options, with HDMI eARC and Optical being the only ones that can output the full 5.1 experience. I'm looking to link this up to my PC. However, I only have my motherboard's...