1.never import a midi song file if you want it to play as it was original composed. OPEN the file.
2. For GM MIDI INSERT the TTS-1 synth.
3. Point all the tracks to that synth ( how this is done in X2 I do not know for sure)
4. Disable you on board sound card, you do not need it and it will cause issues.
5. Go to MIDI options and set you input and output to M audio. ( NOT MS wavetable)
6. Insert a new MIDI track, assign it to the TTS-1 or what ever synth you wish and use a channel not already used by the original tracks. Turn on record and keep the focus on that track ( highlighted)
Hit space bar to practice, "R" to record a new track.
This is all explained in detail if you read the help and tutorials like every body has to do. This is complicated software that needs a lot of patients and time to learn. Good luck.
PS
Also , who started this term "producing" ?? I always thought in the music business the producer sat in a chair behind the engineer and told them and the musicians what everything should should sound like. They normally are not musicians , just a person with good ears and some engineering no how. So is this what people who use music software call them selves now? Just curious because I keep seeing this term used.