I don’t have a Steam Deck, but assuming you’re on Windows:
If you’re not using these exes already, you should try running the “StartFullscreen” or “StartWindowed” executables. RPGmaker by default supports MIDI playback. If the Windows that comes with the SD has MIDI support out of the box, this will use Window’s synth table for playback. If not, you may have to install something like “CoolSoft VirtualMidiSynth” and a “soundfont”.
The reason I recommend trying these executables is because there have been reports with problems regarding MIDI playback support with EasyRPG. You could also try other builds and see if any of those play properly https://github.com/EasyRPG/Player/releases
(We found out that version 6.0 seems to work most of the time, but it’s still not perfect).