It's just general midi, yeah? So 99% of the time that means you want to talk to a MPU-401 UART, which by default means a data port at 330h and a status port at 331h. You get data to/from those ports by using the x86's "in" and "out" opcodes.
There's a quick primer at this link, with sample C code at the bottom: http://midi.teragonaudio.com/tech/mpu.htm
The exact syntax might depend on which compiler you use though.

yeah, I think I'm gonna go with at least C, a naive attempt in QBasic failed, but I forgot 100% of whatever qbasic knowledge I had 30 years ago.

POKE is for accessing memory. For writing to ports you want to use OUT.

I tried OUT in QBasic, it didn't work.

so I moved on to Borland C++ 3.1, and after I finally remembered that inp and oup were in conio.h, I was able to compile this main function, but it still doesn't work (the program runs, but is totally silent. I've tested midi output with other programs like megamid right before running my program, midi output is fine). Maybe you can spot a mistake in my simple one 'note on' midi command?

NEVERMIND
it works, just need to get rid of the while block and make it a one-liner, then perform the note.

thanks for your help!

This will make some sound in QB45 :

1DECLARE SUB WaitMPU ()
2
3CONST MPU% = &H330
4
5OUT MPU% + 1, &H3F    'UART reset command
6
7WaitMPU       '---instrument
8OUT MPU%, &HC9
9WaitMPU
10OUT MPU%, 16  'Whistle
11WaitMPU       '---volume
12OUT MPU%, &HB9
13WaitMPU
14OUT MPU%, 7
15WaitMPU
16OUT MPU%, 127
17WaitMPU       '---pan
18OUT MPU%, 10
19WaitMPU
20'OUT MPU%, 0   'LEFT
21OUT MPU%, 64  'CENTER
22'OUT MPU%, 127 'RIGHT
23WaitMPU       '---note on
24OUT MPU%, &H99
25WaitMPU
26OUT MPU%, 40
27WaitMPU
28OUT MPU%, 127
29PRINT "tone is playing"
30SLEEP
31WaitMPU       '---note off
32OUT MPU%, &H89
33WaitMPU
34OUT MPU%, 58
35WaitMPU
36OUT MPU%, 127
37
38SUB WaitMPU
39
40FOR i% = 0 TO 2000   'change to proper MPU401 ready bit wait
41 a% = INP(MPU%)
42NEXT i%
43
44END SUB