I'm not quite sure what you're up to, but let me give you some feedback about my concerns:
1. You want to do this in ASP.net which tells me that this is supposed to run on the server side.
If the server which is probably standing in some service center even had speakers it would be talking with itself which is of very doubtful value.
2. What exactly do you mean by changing the pronunciation?
Do you just want a different voice like a male of female one or do you want to change the dialect.
Looking forward to your comments!