How to change voice / speech synthesiser to the same as Orca?

[cygnus-glacier]

I have a custom voice for Orca because I don't like the default voice and find it hard to understand. In Orca > Screen Reader Preferences > Voice > Speech synthesiser I have it set to rhvoice (of RHVoice.org) with the language set to en (english). I'd like to change the default voice for ebook-speaker to something else but I couldn't find anything in the manual to do so. It would be helpful to be able to change the voice synthesiser to either be the same as Orca or as a different voice (like a different accent to help distinguish the software's voices).