1. Use gibbed's tools to extract the following files from your SR3 install. All files should be in <your steam folder>/steamapps/common/saints row the third/packfiles/pc/cache:
* misc_tables.vpp_pc
* sounds_common.vpp_pc
* soundboot.vpp_pc
soundboot.vpp_pc must be extracted to its own folder, but I recommend you extract them all to separate folders so you don't get confused later on.
2. Where you extracted sounds_common.vpp_pc, find the media file for your radio station you wish to edit. The filename should be in the format "radio_*_media.bnk_pc". Make sure it has "_media" in it, or else the next step won't work.