Have you imported sound samples (e.g. OpenMPT’s included sound samples at C:\Program Files\OpenMPT\ExampleSongs\Samples) into the song? If you haven’t imported sound samples into the song, it might be worth doing so and seeing if those work instead since OpenMPT by default uses GM.DLS; DirectX’s default soundfont included with Windows for playing back MIDI files. This MIDI sound library might be having issues on your install of Windows (or WINE on Linux). I haven’t had this issue myself though so all this is just my speculation.