For the latter: The RasPi has a 3.5mm jack that you can use. You might have to force it, because it might put the audio through the HDMI port instead. If your sound card has a 3.5mm line input, you can use a 3.5mm cable (male on both ends) to connect both. I used this with my MT32 and my Jazz16 sound card. If your card does not have line input, you can by a small external mixer and use it to connect all your sound devices.

There is guide here on the fotum: Emulating MT-32 on an RPi2

I haven't had luck with latest version of munt (2.3.0), so just used previous one (2.2.0).

How about the midi input? Do I need to buy a Roland UM-ONE USB adapter or I can build a cable to hookup directly to rpi uart pins?
upd: looks like I cant connect directly, but with some circuitry in between https://www.youtube.com/watch?v=RbdNczYovHQ
this makes it easier (and probably cheaper) to simply use UM-ONE

I'm looking forward to playing with this idea using a Pi4. All I need is a 3D printable miniature MT-32 case to put it in.

ahtoh wrote:

How about the midi input? Do I need to buy a Roland UM-ONE USB adapter or I can build a cable to hookup directly to rpi uart pins?
upd: looks like I cant connect directly, but with some circuitry in between https://www.youtube.com/watch?v=RbdNczYovHQ
this makes it easier (and probably cheaper) to simply use UM-ONE

The serial UART can be used along with a current loop converter circuit. This requires reclocking the serial port and you'll lose Bluetooth support.