I've played around with various voice synthesis projects that are available open source on Linux, tons of stuff if you google 'open source speech synth'. There's things like eSpeak which works across windows and Linux. if it can be compiled into a dynamic lib, and it's not classes etc. then it'd be a case of writing a pascal header, which is not too hard if you carefully step through the C header.

A lot of speech synthesis is commercial, there's a market, so don't expect to find a ready packaged solution for all OSes.

There's the microsoft speech API on most windows, but I don't know about headers. OSX probably has a speech api as well.

There's a google speech API that might be suitable for your needs as well