Cool, thanks! I've now set up a virtual EC2 instance locally on my mac, and complied fluidsynth, as you've suggested. I'm still having execution issues, though when I try to call the subprocess to run fluidsynth on the lambda as a subprocess of my python lambda function:
I have no experience with linuxbrew, but it is obviously compiling stuff with it's own lib path. So you need to copy the whole /home/linuxbrew/.linuxbrew/lib/ folder to your AWS instance (under the exact same filepath). Or you could try to make it find the correct libraries via LD_PRELOAD and LD_LIBRARY_PATH env variables.
But maybe it would be better (and easier!) to simply ditch linuxbrew and use standard linux techniques and infrastructure to build fluidsynth and it's dependencies. And you probably don't even have to build all dependencies. I'm sure even the Amazon Linux has some sort of package manager. (Just checked: "yum" should be available). So "yum install gcc make cmake libglib-devel libasound-devel" should give you a working devel environment. Package names might differ, so check with "yum info" or search with "yum seach". Then simply clone the fluidsynth git repo and build it.