> 1) Squirting bytes out of a MIDI-capable serial port to a MIDI device
This is what you do with /dev/rmidiN.

> 2) Taking sound samples in a MIDI file and playing them out a sound
>    card at the appropriate times (as specified in the MIDI file).
This is what you do with /dev/music.  It gives you access to all
MIDI devices in the system and allows you to basically give time
stamped MIDI commands to the kernel.  The kernel then sends them out
at the right time.  You really need to do the timing in the kernel
for any kind of decent behaviour; userland processes cannot time
it accurate enough.  

        -- Lennart