The voice commands are kept inside MBAssets.pk3, in the directory: sound/chars/mp_generic_male/misc/
Create a new .pk3 with the same folder directory as the one listed above, replace all of the sound clips with your own, and rename them exactly as they were before. If the file doesn't have the same name as the one you replaced, it won't work, and the default voice clip will play instead. You can name the .pk3 whatever you want, just make sure you add "zz_z" to the front of the name so that your .pk3 is read before the default.
All the clips must be .wavs exported with these properties: 44100 hz , mono , 32 bit , 128 kbps - otherwise they won't work.
You will also want to add some effects and filter the voice. Because it is being exported in mono, you will need to increase the volume of your voice clips, and if you want them to be audible in game they will need to be hitting 6db.
I recently helped create a voice pack and it is quite a bit of work, so good luck and let me know if you have any troubles.