Is simple voice synthesis (just digits 0-9) possible with Cortex-M0+ with 4kB flash?
While this may not be a complete, independent and ready-to-use answer, but I think you can get some neat ideas, implement (or port) it on Cortex-M0. Here I assume that from a computation power and resource standpoint, a Cortex-M0 has more to offer, than the popular Atmel 8-bit AVR (ATmega328P) running on an Arduino.

Here are 2 projects, that manage to use the PWM pin of Arduino and an RC-filter circuit to play out synthesized speech. Of course, we are not looking at hi-fidelity audio, but something that is recognizable. Also do note, that apart from the need for a PWM capable pin, your micro-controller might be very busy during the synthesis, so much so that, it might spend most of it's cycles doing it. Software PWM would put further strain.

Now for the 2 projects:


Cantarino
Port of Webbot's speech synthesis to Arduino


PS> Personally, I've not implemented them, but looked at them for a project.