Linux does not come "pre-installed" on any Chromebook. Whoever gifted the Chromebook to you put it there. And it's not going to work WELL, in any case, because Chromebooks were never built to run anything but ChromeOS. I doubt many Linux experts can get LMMS to run on a Linux-ified Chromebook without days to weeks of tinkering. You'd have better luck with LMMS on a Raspberry Pi than on a Chromebook. (Old-timer Unix joke, "Here's a nickel kid - get yourself a real computer.")

I love my Chromebook (for travel). Everything just works. But I leave it alone as far as trying a different OS. Not all Chromebooks can do that at all without flashing the BIOS with custom code - and in my experience with step-by-step computer instructions, the sixth step always fails. Avoid step-by-step instructions over 5 steps long - they never work.

The secret command to make an AppImage file executable in a Linux terminal window is "chmod +x <filename>". Then to run it from the terminal window, "./<filename>".

And you don't have Jack (lol). Installing and using Jack is non-trivial, and the on-line docs for it are a nightmare - you'd know it if you had it installed. Besides that, what Jack would do for you doesn't really apply on a Chromebook. How many audio devices do you have? One. How many possible outputs does it have? One. You don't need Jack. IMHO Jack is for people doing home studios.

zynaddsubfx is one of the "built-in" synths in LMMS - but it's the only one for which LMMS will not provide a native LMMS interface. To get to the "real" interface of zasfx, it has to fire up the external version. And if you do that - good luck. It is... user-vicious. Stick to the presets.