The code should actually be compatible with GMS1, since I haven't used any GMS2-specific functions, but given GMS 1.X discontinuation, I didn't know if somebody wanted it. I'll try to make a GMS1 compilation when I can.
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Well, in that case, I understand how it could be misleading. I'll try to be more clear in future posts. I didn't mean to be misleading in any way, but sorry for the confusion.
Aside from that, the CRT filter I've used in Project Bullets is a free asset from the marketplace. I've searched the link for you but couldn't find it. It was xygthop3's CRT Pixel Shader. And for the music thing, I'd be enchanted to work as a music composer (I'm actually building a portfolio for that). If you want, you can send me an email to email@example.com and talk it a little bit better :)
Actually, I do not know what you expected. First of all, GMS is unable to create dynamic sounds on his own, given that it doesn't even have an audio processing engine, so the only way to work with something like this is with pre-recorded audio. Second, it's a basic engine (stated in the post) which not only works with audio sync groups but it also controls things like decays and fade time. Third, the reason that this thing is paid it's almost exclusively because of the music (which is reusable in any project, only needing credits for composition), and finally, it's ready for any user to understand it quite quickly, given that sync groups are not a common theme in the tutorial scene for GMS. 95% of the code assets you can buy are actually pretty simple scripts or small engines made for the comfort of the more novice users.
That statement stated, I'm actually eager to see if you can give some tips to update this with some other things.
This code let's you put different adaptive tracks with ease, and comes with one song (3 tracks) included. As I've said, I'm also a composer, so you can talk to me if you need something on the style.
Thanks for the tips on the title and post, I've already added them!