Killing the mt32d restores normal work of aplay. So, it's impossible to stop mt32d playing the silence?
Regardless the silence playback, mt32d plays incorrect non-linear tempo. May be it depends on sound font (your installer downloaded it). Also i see "Output buffer underrun" message. Not sure who exactly shows it - alsa or mt32d. mp3 playback doesn't produce any warnings/errors.
Obviously not impossible as they are open source, but looking at the code of FluidSynth and MUNT there is no easy option - and they both exhibit that behavior.
I think it would be best to kill MUNT and Fsynth in the uartmode script for the NONE/PPP/Console options.
Also MUNT has no soundfont. That is for FluidSynth.
MT-32 works like an analog synth with PCM samples only for the attack portion of the waveform. The closest thing it has to sound font is the ability to upload new PCM samples via MIDI sysex messages, which is what a lot of games (especially Sierra) do.
I think the tempo of MUNT got messed up when I recompiled it with what I thought were better optimizations. I am going to recompile it and then we'll see if the tempo is any better.
Ideally you should be using MT-32 specific MID files for testing with MUNT. or at least the --port 128:1 which will tell it to use a semi quasi GM sound bank mapping.
I did get the "output buffer underrun" (with MUNT) but that was when I was trying to write a a sound file on /media/fat and never with the /dev/MrAudio