Well, the callback function isn't an exported function so you cannot tell by it's name.
However, by reading the MSDN library you can see that the callback function is defined as void CALLBACK and CALLBACK is simply a definition for _stdcall(which is the standard calling convention)
According to MSDN, all Win32 API callbacks are always using the standard calling convention and so does all Win32 API functions.
That's why I'm a bit confused why your midiInOpen function works even though you haven't added "win32" to it.
Have you tried calling "midiInOpen" several times? Because sometimes you won't get an error unless you call it more than once.
However, by reading the MSDN library you can see that the callback function is defined as void CALLBACK and CALLBACK is simply a definition for _stdcall(which is the standard calling convention)
According to MSDN, all Win32 API callbacks are always using the standard calling convention and so does all Win32 API functions.
That's why I'm a bit confused why your midiInOpen function works even though you haven't added "win32" to it.
Have you tried calling "midiInOpen" several times? Because sometimes you won't get an error unless you call it more than once.