Thank you for the IRP file. It looks as I expected. I made a mistake last night in my interpretation of Function 5.
When I originally examined Function 5 last night, I incorrectly transposed the device and function.
Function 5 above is indeed correct and what i expected it to be.
To use the NEC1.irp file to generate output matching the format of Jason's above, you should change the device line to:
Device=2.253
Note, however that if the ".253" is not present, MakeHex automatically supplies the binary 1's complement of 2 which is indeed 253. (more information that you likely needed or care about).
If the protocol is as we expect, Jason's codes should work. The codes you had originally should have also worked if you were using the code data from the correct function numbers.
Jason has provided the pertinent functions you need. Have a look at VOLUME DOWN and see that it is function 144 (0x90). Note that I pointed out that you needed to use function 0x90 in my original post.
Hopefully, you are not frustrated due to the extra round-trip here but, instead are happy that you learned a bit about hex-to-decimal conversion and also how to use the wonderful tool MakeHex provided by John Fine.