Music from a trailer(not apex legends) starts playing everytime i open the game.

Thanks, that's so strange. Do you have anything within your Steam Properties > Launch Options for the game?

If you swap the game to windowed and open the Volume mixer, does it show the sound coming from the game app or another one?

To open the volume mixer right-click the speaker item in the tray at the bottom right of your desktop, then choose the Open Volume Mixer button.

Here you can adjust relative volumes per active program, and it can also be used to track down a problematic audio output.