Musical Note Length Calculator in C
Your calculations are all off by a factor of 10: one minute is 60000 milliseconds, not 6000.  Instead of explaining your 6000 using a comment, write it out in the code, and the compiler will figure it out for you. That calculation will not cost you any runtime performance at all.

I don't recommend that design, defining one function per type of note.  One function should cover everything, I think.

Since you use printf() and scanf(), you should #include &lt;stdio.h&gt;.

It is customary to put main() at the end, so that you don't have to declare the functions it uses in advance.

#include &lt;stdio.h&gt;

#define SEMIBREVE       (1.0)
#define MINIM           (1.0/2)
#define CROTCHET        (1.0/4)
#define QUAVER          (1.0/8)
#define SEMIQUAVER      (1.0/16)
#define DEMISEMIQUAVER  (1.0/32)

int millisec(int bpm, double note) {
    return (int)(
        60      /* seconds */
        * 1000  /* milliseconds per second */
        * 4     /* crotchets per semibreve */
        * note
        / bpm
    );  
}    

int main() {
    int bpm;
    printf("Enter tempo (crotchets per minute): ");
    scanf("%d", &amp;bpm);

    printf("Semibreve would last for... %d milliseconds!\n", millisec(bpm, SEMIBREVE));
    printf("Dotted crotchet would last for... %d milliseconds!\n", millisec(bpm, 1.5 * CROTCHET));
    printf("Demisemiquaver would last for... %d milliseconds!\n", millisec(bpm, DEMISEMIQUAVER));
}