Hi, I am not an expert on the topic and would suggest the midi.org or the Wikipedia articles for General MIDI and MIDI.
I looked at the spec for the k750 on developer.sonyericsson.com and it says that the media player supports "G-MIDI (level 1 with 40 voices polyphony)" which is the same as the k800.
There may some Java ME specific documentation on the site. I would try saving it as MIDI1 w/o SysEx. I would also try out a small file.
This is what we did when we had a problem playing midi files on certain devices. We used a Java program to convert our type 1 files to type 0 and packaged only type 0 files with the application. We also ran into problems with the size of the files. Some larger midi files would not play.
I found this converter searching today:
http://www.jsresources.org/examples/MidiConverter.html I posted the code we used here:
http://www.worlddeveloper.org/www/forumtopicview.html?fid=185&categoryId=36&fpn=0 I use Reason 2.0 (
http://www.propellerheads.se) and sometimes Adobe Audition.
You may get a better answer on the SE site.