Using custom MIDI instruments
Choose your instruments
Since the MIDI standard does not define how the instruments sound, there exist special sound banks (hereafter referred to as "sound fonts") which can be used by synthesizers. There are different sound fonts in several different formats, and created with different goals in mind; a sound font for orchestral rendition of classical music will not have the same priorities as a sound font made for rhythm and blues songs. The software synthesizer you want to use may limit your choices.
See the sound fonts page for a few pointers.
Choose your synthesizer
ZDoom offers several different devices for MIDI playback. They have different capacities, advantages and drawbacks. The format of your sound font of choice may limit your options.
See the software synthesizer page for further information and instructions specific to each synthesizer.
Once you have set up your sound font of choice, and configured the synthesizer to point to it, you can select it in the sound options menu and ZDoom will use it, unless a different choice was selected for the current song in the SNDINFO lump.
In addition to what ZDoom provides you with, you can try installing additional MIDI drivers for your OS. ZDoom should detect them and let you use them.