I was programming a bit in SDL, when I saw the program print
"open /dev/sequencer: No such device".
So I started to investigate a bit. And, yes, there was no such
file as "sequencer" in /dev. I tried to open a MIDI file in KMid
which just said that /dev/sequencer was probably being used by
another program, while it was unavailable. But I already knew
that the file didn't exist.
Now, how do I solve this? I've played MIDI files (many) times
before, without any hazzle (both in Windows and other versions
of Linux). Right now I use Debian 3.1r0a Sarge, and I have an
Ensoniq Soundscape (es1370) soundcard, which seems to be
correctly configured, the module is loaded, and (recorded)
sound works. What do I've got to do?

Regards,
Stefanos

If your soundcard can play midis via hardware surely you'll want a better solution for your problem, but if your soundcard can't do it (or simply you want to be able to use any soundfont for playing and be able to apply cool effects) you can use a software synthesizer like fluidsynth (http://www.fluidsynth.org/) also available in debian's repository as well as it's front-end qsynth.

I had the same problem
'pmidi -l' was not finding the midi ports at all, and gave a similar message about no /dev/sequencer, and I got the same error message from kmid.
This is in debian (libranet), but there had not been this problem with the same hardware in other distros.
I ran 'modconf', then selected kernel/sound/pci/emu10k1, and found three emu10k1 modules, but only one was enabled. I enabled 'snd-emu10k1-synth'.
then it worked