Probably because it's a type of music file that is cued to play once without interruptions or loops. So all it is told for the intro music is to play... and that's it.

yeah figures, and thank you. the good way to mute sounds is, indeed, to mute it and keep the length just like that. but the file size still may end up very big, when you have to mute a lot of sounds, even if it's just silence.

I figured to experiment. this may be useful to save up storage, using blank.wav

this addon doesn't screech. probably? so far hasn't.

like let's say you mute some survivor voice line, and shorten the length of it. the muted part will be muted. but at the time point, where there's supposed to be something, noise or silence, the part which you shortened; is not present. game compares length of the file and the soundcache's logged length. (I think??)
and then, can't figure out what to play if there's nothing, not even silence. so it screeches. I guess...

however, this, for reasons I don't know, still works, even if it has length shorter than should be. how? idk. maybe because blank.wav is like 1 frame long? or maybe cause it's music? but thanks

Well, first time seeing this. Haven't tested yet, but if I do hear glitched noises at some point during which an intro would have been present, I'll let you know. All I do know is that a set of working mute survivor mods said that the modder went in and muted all the survivor lines individually without changing the file length or other stuff. Just simply set the volume in editing to 0, and it works a treat.

good for you
and well thanks, I'm plentifully relieved

Also, I've been meaning to tell you that I love your custom thumbnails and preview screenshots/videos. I can tell you put a lot of effort into them.

Appreciate the extra effort.
I've been using this mod for a couple of days, and I didn't experience any screeching either. I should try the method in the near future as well. It could come in handy especially when making horde music mods, because you have to silence a lot of audio files for those, and they end up causing the mod file size to be like 194 MB or more.