As in, you just want to decrease the speed of the music overall by default? For that, in the text file of the music, lower the number next to the "t" to as how much you want it to be.
https://bin.smwcentral.net/u/40814/Overworld.txt i've searched into the entire txt file but i didn't find it!
It's in this part, highlighted in white: Code;Channel 1 #0 $f4 $02 / w199 t34 $ef $f9 $00 $00 $f1 $02 $1e $02 $f2 $18 $0e $0e You must've overlooked it.