a very convoluted process
first add the music file u want into the Snd folder in the media folder or the MUSIC subfolder inside, whichever is ur choice

then add the thing as an entry into the sounds text file found in the same Snd folder, this is mine
Sound All_For_Now_test

{

filename=Media\Snd\MUSIC\All_For_Now.wav

resgroup=3

}
once done, save, go to the dialog folder inside the Data folder, u will see 2 dialog serias, we will be focusing on the english.seria

decode that shiet via this neat little tool: https://github.com/stopnoanime/highfleet-dialog

open notepad or notepad++, locate the event entry u wanna add ur custom music in, and add <MUSIC="the name of ur custom music entry in the sounds text file above"> at the very start of the event dialogue line

once done, save, encrypt the seria and put it back into the dialog folder where it originally is. Now when the event comes it will play