For editing single sounds, this section in the wiki covers it: https://github.com/thane98/BCSAR-View/wiki/Editing-Sound-Sets
For the error, have you set up any tools do wav <-> cwav conversion? The tool doesn't have a built in way to do this yet, so you need to configure another program. Wiki: https://github.com/thane98/BCSAR-View/wiki/Setup#wav---cwav-conversion
Subtitles are probably in the m folder of the ROM. You could edit these directly in Paragon, but for this it would be easier to use FEAT: https://github.com/VelouriasMoon/FEAT/releases