Main trouble is a fact, that 'sgt' also is short for 'sergeant'. So, Google returns news, military sites, Beatles fanclubs etc.
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I couldn't find any specification for *.SGT. MSDN said, that *.SGT is a MIDI-based segment file. I trust it, info in the middle of SGT-file looks like MIDI information (checked by *.MID-specification). But I don't understand meaning info in the headers of SGT.
So, I founded:
1. Each SGT-file starts from RIFF (in hex, 52 49 46 46)
2. Next 4 bytes points to 00 00 00 00 00 00 00 80 at the end of file
3. Then segment description follows by DMSGsegh
4. Track description follows by DMTKtrkh
5. Info in the middle of SGT-file looks like MIDI-information