With Liuux it should be libc.so, but windows it is 'msvcrt.dll'
[...etc...]
I wouldn't call these external libs as such, linux and Windows are written in C, so it is ever present awaiting instructions.
But some folk will disagree.
Balderdash! You don't need either, in Windows or Linux. In Windows, for example, you can use a simple system call to
sndPlaySound and, probably, something similar in Linux, MacOS, etc.
LIBC should only be used when you
aboslutely need compatibility with Kylix. And almost the only use of MSVCRT (that is, the MS Visual C Run-Time) is to be able to mimick exactly some piece of Visual C code. Both
are external libraries, with little to do with the system itself.
So yeah, "some folk will disagree"