QMK song player is a web app that reads QMK song code and plays it in your browser.
If you have a QMK keyboard with a speaker, you can use the audio feature to produce these songs from your keyboard (how to add songs to your keymap). Why do this? Because it is fun! You can play a song when the keyboard starts up, or more functionally, play a little melody to indicate events like when setting the default layer.
QMK comes with a good number of predefined songs out of the box. Here are a couple to whet your appetite:
More songs are in quantum/audio/song_list.h. You can also edit the code and write your own songs, see QMK song syntax for how to do it. Enjoy!
Yes to combat music, excellent idea! If you can play it by ear or find sheet music for it, it can potentially be coded note by note in QMK song syntax.