Port midi requires a few kernel depends that I missed in sunrise. CONFIG_SND_USB_AUDIO is definitely required and CONFIG_SND_VIRMIDI is probably required. I wont be completely sure until I get home tonight. There should probably also be a warning if CONFIG_SND_SEQUENCER is compiled in as a module. See Bug 406587
CONFIG_SND_VIRMIDI is NOT required.
You write about sunrise, but the ticket was assigned to the package in the official tree. Is this still a problem in media-libs/portmidi-217-r1 from the official tree?