The loop points need to be edited. This is found within the bank files in the NX64 folder where the sound is. However, the struct needs to be known first in order to figure it out. I suspect that we need to edit either Init.bnk, or BGM.bnk.
These are files generated by Wwise 2017, so any tool which can edit loop points for files generated by Wwise 2017 should suffice.
See, I was overcomplicating my life by looking at Hex edit
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download sound editor 2018 and wwise
then convert any music via wwise, open stream.pck with sound editor and find the music you want to replace.
huh? can it be that simple?