Sounds like in part it was because the SDL2 libs I'm using are compiled without some bit of functionality?
Yes, from your strace output, it seems they were simply compiled without SMPEG support.
Or maybe adapt at runtime depending on whether libsmpeg.so is available, and I was missing it?
Hard to tell... indeed SDL2_mixer may open libsmpeg2.so as a shared object at runtime, if it exists and if MP3 files have to be played. Do you have such a file at the same location as "libSDL2_mixer.so"? But it may also be possible that smpeg2 was compiled into SDL2_mixer directly -- it's included as an external library in a sub-directory of the SDL2_mixer source.
At least for the Android version of SDL2_mixer, smpeg2 compiles as a separate shared library that is loaded at runtime. On the Mac, it seems to be compiled directly into SDL2_mixer.dylib.
That reminds me that I have a similar problem with the last SDL2 versions with Android: I can play WAVs and MODs, but no MP3s, even though the debug log tells me that "smpeg.so" was loaded. So I continue using an older version of SDL2_mixer/smpeg2 for the Android version for now...
You're saying that when you build the binaries, you will package with SDL2 and smpeg .so files.
Yep, right. Therefore the packages are self-contained and contain everything they need to run on any Linux system that is not older than around five or six years. (Excluding things like X11 and C libraries, of course... I didn't want to be THAT self-contained...
But this is not suitable for distros like Debian / Ubuntu where they're going to build their own.
It doesn't matter, as the libraries included in the R'n'D package do not collide with potentially already installed system wide SDL/SDL2 libraries.
In the past, I never had a Linux system with 100% working SDL/SDL2 libraries with all required/dependent sub-libraries from the distributions repositories. (I've used various SLS, Slackware, SuSE, Debian, Ubuntu and Kubuntu Linux distributions in the past 20 years, and so far I always ended up compiling my own set of SDL, SDL_image, SDL_mixer, SDL_net, mikmod/modplug and smpeg libraries, as the distribution's variants were either non-existing, incomplete or outdated.)
Nevertheless, any distribution's package maintainers are free to bundle R'n'D with all required libraries as they like, so in an ideal world my download package would only ever be a last-resort fallback solution for esoteric distributions, as everybody's using the pre-packaged version installed from the Ubuntu Software Center application (or how it is called).
(At least for Ubuntu and R'n'D 18.104.22.168, this already seems to work fine... But this version is over 6 years old, and the last stable release is over 3 years old.)
I should investigate and make sure that binaries built out of their build env will end up with the right set of everything. And then bugreport to them "update your RnD" -- when 4.0 final is final.