Aloha L,
In the MIDI file (usually near the beginning) there is an inserted MIDI message (event)
called: ‘Program Change’, telling Halion Sonic to bring up the Poly Synth patch.
Some times there are more than one inserted along the file.
You need to find these messages and delete or modify them
to the sounds that you want HS to play.
However I do not think you can do this in HS as a stand alone app.
But
If you load the MIDI file into any version of Cubase.
you can then use the ‘List Editor’ feature to find
and eliminate/change these messages
HTH (hope this helps)
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