MIDI files are just a set of notes pitch and duration plus some additional settings or instructions. Technically speaking one can’t “play a midi file” one can read it and synthesize sound based on it. This is covered by stuff called Software Synthesizers (we aren’t speaking of Hardware Synthesizers, most of which use Software Synthesizers inside anyway nowadays :D). That means that if you play your midi file in different Software Synthesizer it would sound differently, or may not play at all. Moreover, Linux/Android don’t have a Soft Synth preinstalled by default - it should be installed manually by the user.
So the only solution to is to integrate a software synthesizer into Castle Game Engine or your specific game project. This is not something impossible. There are opensource midi synthesizers like Timidity or FluidSynth.
However, you should notice, that those are of relatively poor quality, even compared to trashy GM Microsoft Wavetable Synth. Therefore, a much, much better idea would be to use free (as in free candy) or even paid synthesizers (like trash-but-has-everything Vir2One VSTi) to convert your midi file to WAV/OGG and play it from there. We are not in 20th century anymore - we can afford 5-10 additional megabytes of HDD space to provide a (much) better sound quality. Yes, it’ll take some time to get used to some music software (like LMMS), but it dramatically pays off in quality. And, yes, I couldn’t find a proper free soft synth for myself, not even a chiptune - still my trash Yamaha PSR230 sounds better than that, I miss Korg PA80 badly
It’s also possible to create a soft synth based on “pure CGE” - it’ll be tricky to get proper polyphony (at least 32 voices) though. Plus I’m afraid of possible memory implications, e.g. one of my favorite Native Instruments Concert Piano VSTi was a few (dozens?) Gb. This looks like a good solution for “Android virtual piano keyboard” but not for real-time synthesis inside a game.