Can someone explain what this jumper does? Is my understanding correct that it controls the emulated mpu-401 interface vs. a true MPU-401 interface?
More-or-less. With the MFBEN jumper open, the card is physically unable to request/generate the non-maskable interrupts used by AWEUTIL to "re-direct" MIDI data to the EMU hardware (when used as a TSR).
1. If this is correct then would you address a DB on the AWE32 using the I/O of the SB AWE32 for the MIDI device?
MIDI daughterboards are commonly accessed through the SB's MPU-401 interface ports, addressed at 330-331h by default.
2. Would this disable the joystick port?
No.
Or would I still be able to use the joystick port and use the DB at the same time?
Yes.
3. Does this hsut off the AWE portion of the AWE32?
No.
That is, if a game has drivers for the AWE32 would closing the jumper make that driver usless...
No. Most games having an AWE32 option for music playback will access the EMU hardware registers directly.
