Hello again! I took some time to start working on the audio part of the program, as well as using the existing UI code to manipulate stuff.
Here's the link to the build.
Running it will open a file dialog, to let you select a music file (I have confirmed that MP3 files work so please try not to select anything else :) ).
On the left the 2 sliders are used to change the position of the song (green-ish slider), and the volume (blue slider). The blank space on the right is actually a button, and pressing it should pause/continue playing. The sliders have (slightly) different behavior: the position slider will only affect the song's position when the left mouse button is let go. The volume slider will change when the slider is moved.
The program is still pretty buggy, but if anything starts to really not work, please tell me! But in general, any observations and criticism are all welcome!
Next up, I'll be working on making the layout a bit more flexible, and adding some more features to the audio component (changing the song at run-time etc...).
Thank you for reading!