Having spent a couple of hours decoding the output from a Holtek HT600 encoder (in a Micromark wireless burglar alarm system), using the voltage divider / sound card / audacity method, I decided to post the specs to save anybody else the trouble...

01 01 0 - 5 bit header
01 01 01 01 01 - A0 A1 A2 A3 A4
01 01 01 01 01 - A5 A6 A7 A8 A9 (note A5 doesn't exist and is always 01)
01 01 01 01 01 - NA AD11 AD12 AD13 AD14
01 01 01 - AD15 NA NA

As noted, there is no A5 pin on the chip - this results in 01 always being present in this field. I'm not sure what the final four bits do, they always seem to be 01.

Hope this helps. I'm hoping Bruce's code will decode this - otherwise I've got the info to write my own :-)