I don’t think it’s rude at all….you have to still support yourself when you volunteer . You aren’t Melinda Gates or Trudie Styler (I’m guessing you’re not…anyway.)
Just word it dipolomatically….: “Hello, I applied for one of your positions and I would love to do this work…..” then just talk about the position, and what it entails. They might actually offer the information in the conversation. Or they might just say, “Do you have any more questions?” And you can say, ‘Yes, I understand there is a bit of a stipend provided? What would that be as I would just like to be able to know what I will need to provide for myself additionally to do this work…because I really think this position is something I would really enjoy.”
If they don’t seem to be forthcoming toward the end….just say, “Oh, yes, one more thing…you mentioned a stipend…? ” Usually that will work.
Write down both of these options, put them by the phone…in case you get nervous and forget what to say.
All the best of luck to you….and I think it’s great you are volunteering!