I put together some instructions for adding a MIDI connector to a Dreamcast modem. The MIDI output is controlled by the AICA sound processor, and it requires special software support to use it. My alternate ScummVM backend for Dreamcast might be the only software that supports it, but there isn't much that supports the official Dreamcast MIDI Interface Cable either .
For a while I was using this instead of a MIDI Interface Cable because they are impossible to find and because it ties up the serial port. This mod might be slightly easier than making a homebrew MIDI Interface Cable too.
A lot of the games ScummVM supports have soundtracks composed specifically for certain vintage synthesizers like the Roland MT-32 and SC-55, and there are probably other Dreamcast ports that could benefit from MIDI output. Doom had a MIDI soundtrack, so I'd like to add MIDI output to one of its Dreamcast ports one of these days.