Adding music and sound
This is a huge topic but here are some tips if you would like to create the music and sound yourself (instead of buying or finding free tracks).
Generally: When recording audio, get the sound level as good as you can. (See Tip.) Practice with some app or software where you can see a level meter. A too low level or too high (when the signal hits the max level) is just bad.
To add music you could either record live instruments or create a music track by midi.
There are free midi players out there but I only want to recommend software that I have personal experience of and that I'm sure has no so called bloatware or worse.
I have been using Cubase since ages. However, I decided to create a super simple free midi player so I could at least recommend one free midi player. It is called Midi Pad Light Version - Midi Player and Audio Converter and can be downloaded from Microsoft Store. (Icon for Microsoft Store is often found down in the taskbar on Windows.) The free light version of Midi Pad can also capture audio and create midi notes, so you don't really have to be able to play an instrument, just sing it. Bare in mind though, it will need editing, capturing audio is not easy, we humans (and also analogue instruments) don't create clean tones as synthesizers do. You should be able to find a free midi editor somewhere (consider open source from safe site), however if you would like to be able to create music yourself, a proper DAW like Cubase is highly recommended.
Recommended free software: Midi Pad Light Version - Midi Player and Audio Converter
Suggestion: Consider paid software if you'd like to create more advanced musical tracks.
Adding simple sounds, like environmental sounds or sound effects or simple instrumental tunes can be quite easy. You can for example record them with your mobile phone or Audacity and edit them in Audacity, a free and easy to use audio software. Audacity has quite a lot of editing tools like low and high pass filters (to remove noise) and create effects.
Recommended free software: Audacity