All executable files of notable size have been compressed with UPX and have to be de-compressed before they can be modified in any meaningful way.
As of this version, the output port for Tandy sound is hard-coded in a word variable rather than in the code itself. If you have an ISA card that provides Tandy sound capabilities at a different output port, you only have to find and change the correct C0 00 word.
The palette in PX3_VGA starts with 00 00 00 AA 00 00 AA 55 (offset 0x8546), while the Tandy lookup table in PX3_MTDY starts with 00 44 66 17 33 11 55 77. The upper four bits of every byte in the Tandy lookup table contain the closest match, the lower four bits contain the currently unused second-closest match. Attempts to manually tweak the LUT to improve the visual appearance of the 320×200 Tandy mode are very welcome.
The game artwork uses ZX7 compression. Once de-compressed, the file formats of Planet X3's graphics and map files are trivial.