I'm used to specifying an IRQ for a sound card but what does it mean in the context of a MIDI music card setup?
Back in 1984, long before the Sound Blaster existed, Roland introduced the MPU-401 MIDI interface that enabled external MIDI-equipped electronic music hardware to interface with a PC.
It was only later on that game companies (like Sierra On Line) used the Roland MT-32 with their games when this became a more notable feature in the gaming industry.
When the original MPU-401 was introduced, the IRQ (interrupt request) of the interface card had a default IRQ of 2 (it was actually using IRQ 9, which is "linked" to IRQ 2). Later on, as new revisions came out, Roland made the IRQ changeable.
When the Sound Blaster was introduced in the late 80's, it used the default IRQ of 7. This was later changed to IRQ 5 due to conflicts with the LPT1 port, which also made use of IRQ 7 (makes we wonder why Creative chose IRQ 7 in the first place).
It was only around 1992 that sound cards were introduced with built in MPU-401 UART MIDI interfaces - before that, you required a dedicated MPU-401 MIDI interface (these were normally the "intelligent" mode types) in order to play back and to listen to Roland MT-32 music.
Since the original MIDI IRQ was different to your Sound Blaster's IRQ, some games will prompt you to specify the correct address and IRQ since, in it's original form, the MIDI interface card and the sound card were two separate devices, requiring their own addresses and IRQ's.
Some games, as you've mentioned, is able to "detect" it or, "assuming" it's on the default IRQ of 2 (9). The latter will therefore not always work if you change the default IRQ on the MIDI interface card.
So, Micropose actually made their games more "configurable" by allowing the user to change their hardware's setup and then specifying to the game where to "find" and "talk" to the hardware.