Is it possible to use some C source code in iPhone project?

I found a speech synthesizer that supports many languages (http://espeak.sourceforge.net/index.html). But the source code that is available is written in C.

What can i do to make it work on iPhone?

Objective-C is a strict super set of C. Which means you can use any C code in your Obj-C project.

Yes, as long as it's been ported to the LLVM C-compiler. You can create a statically linked framework out of it, and then link into your project.

Apple does not allow dynamically-loaded frameworks.