For AWE based sound cards, applications and games are expected to use the EMU8000 chip directly (via the default port of 620h).
However, for games that didn't support the EMU8000 synthesis chip natively, Creative wrote a tsr called AWEUTIL.
AWEUTIL uses Non Maskable Interrups (NMI's) to "catch" I/O events on port 330h (the standard MIDI port), and then reroutes them to the EMU8000 synthesis chip.
This works only with Real Mode games, but not every one.
Therefore, Real Mode DOS games requires Aweutil to initialise your sound card's General MIDI mode (with the /EM:GM parameter).
You would then choose General MIDI in the game's setup menu (under the music option).
Just an additional note: For the above mentioned MPU-401 AWEUTIL emulation to work, the card MUST have the "MFBEN" jumper (JP4) set as this is responsible for generating NMI's.
In order to hear General MIDI sounds with Protected Mode games (games that load a DOS extender like DOS/4GW), it must support the AWE32/AWE64 natively (i.e., directly in hardware) and doesn't actually require Aweutil (you can then just load Aweutil with the /s parameter to initialise the EMU8000 chipset).
You would then normally choose AWE32 (not General MIDI) in the game's setup menu under the music option.
Unfortunately, with this option, you're stuck with the standard 1 MB General MIDI ROM sample set that is on the sound card (and some of the instruments, unfortunately, do not sound that nice - the trumpet being one of them).
I've had some luck with Protected Mode games by booting up into Windows 98, loading a custom soundfont and then starting a DOS Command Prompt (full screen window).
I can then select General MIDI (not AWE32) in the game's setup menu and will then hear the custom soundfonts that I've loaded in Windows.
If you want to play your MIDI data via the AWE's external MIDI port, be sure to insert the "SET MIDI=SYNTH:2 MAP:E MODE:0" command into your Autoexec.bat file
SYNTH:1 routes MIDI data to your internal EMU8000 synthesiser, whereas SYNTH:2 routes MIDI data to the external MIDI port.