A .kss is an MSX game's music with the game removed: a Z80 sound driver and its data, and sixteen bytes saying where to load it, where to call, and which chips the driver talks to. In Sega mode the same container holds a Master System or Game Gear rip. After the header comes the init data, loaded at the load address, then any number of banks the driver switches through, 8 or 16 KB each. A file may end inside its last bank; the player fills the rest with zero, and nearly half of them do.
KSSX. The extended magic adds an extension the byte at 0x0E sizes, sixteen bytes in every file seen, and the init data begins after it. What the extension holds is not documented; it is zero in almost every file.
The bank byte. Bit 7 set means 8 KB banks, clear means 16 KB; the low seven bits count them. Read the other way round, a third of real files stop adding up.
Zero means the PSG alone, the AY-3-8910 every MSX has. The higher bits are left as they are found.
Four bytes at zero, one of two spellings.