@Pentium: Thanks for the tips. This solved my p:bios problem as well. I've got a 1.4/1.5 xbox (Sept 03, I think; 4920/5101).
I was further able to relocated the evox install directory and further modify the xecuter bios by doing the following.
In the xbtoolConfig folder, move "multi.ini" to "More Configs" and move "More ConfigsDebug.ini" back to xbtoolConfig.
Edit the Debug.ini and copy the RC4 key from boot.cfg (from PBL1.3.5i_signed_audio_exploit) to the RC4_Key= line.
Then, you should be able to open and edit 4979.67_bfm_256k.bin, and start evox from e:evox as described in this tutorial.
What I don't understand is what is the difference between 4979.67_bfm_256k.bin and 4979.67_256k.bin, and how does one get from one version to the other.