Holtek manufacture a couple of chips which require only a few external components which can perform the task you require. It is not difficult to implement the type of voice manipulation you are after with a microcontroller. I don't think speed is an issue. The Dallas DS80C320 should be up to the task, if not, the DS80C420 should be. Implementing echo algorithms is a little hungry on memory, and to achieve echos of any reasonable length at a good sample rate (>11025KHz in my opinion) is going to be difficult with even 64K RAM. Does your application really require a microcontroller? If so, what other specilaist functions do you require it to perform?

Matt.