Well, I have no idea how easy it would be to use a C++ library in this particular BASIC dialect, but most of the time it's easier to stick to the C interface, which is explained in the docs
. Currently most functions lack documentation, but that's not much of an issue as they generally do exactly the same thing as their similarly-named C++ counterparts
OpenMPT also comes with a simple C example to demonstrate the calls necessary to open and play a module file.
And one other question: does Open MPT read & play sample audio by the buffer amount or by some other variable?
You will have to specifiy which "buffer amount" you are actually referring to. libopenmpt does not do any audio playing on itself, that's the job of the user. And as such, it will render any amount of samples in one go, but it should be kept between a few hundred and few thousand per call to keep memory copying overhead and latency low.