it is in the masm32 package, named winmm.inc/lib
Hutch puts all the constants in windows.inc (which includes winextra.inc)
the link you gave does have prototypes
1506 UINT WINAPI mixerGetNumDevs(void);
1507 UINT WINAPI mixerOpen(LPHMIXER,UINT,DWORD_PTR,DWORD_PTR,DWORD);
1508 UINT WINAPI mixerClose(HMIXER);
1509 DWORD WINAPI mixerMessage(HMIXER,UINT,DWORD_PTR,DWORD_PTR);
1510 UINT WINAPI mixerGetDevCapsA(UINT_PTR,LPMIXERCAPSA,UINT);
1511 UINT WINAPI mixerGetDevCapsW(UINT_PTR,LPMIXERCAPSW,UINT);
1512 #define mixerGetDevCaps WINELIB_NAME_AW(mixerGetDevCaps)
1513 UINT WINAPI mixerGetLineInfoA(HMIXEROBJ,LPMIXERLINEA,DWORD);
1514 UINT WINAPI mixerGetLineInfoW(HMIXEROBJ,LPMIXERLINEW,DWORD);
1515 #define mixerGetLineInfo WINELIB_NAME_AW(mixerGetLineInfo)
1516 UINT WINAPI mixerGetID(HMIXEROBJ,LPUINT,DWORD);
1517 UINT WINAPI mixerGetLineControlsA(HMIXEROBJ,LPMIXERLINECONTROLSA,DWORD);
1518 UINT WINAPI mixerGetLineControlsW(HMIXEROBJ,LPMIXERLINECONTROLSW,DWORD);
1519 #define mixerGetLineControls WINELIB_NAME_AW(mixerGetLineControls)
1520 UINT WINAPI mixerGetControlDetailsA(HMIXEROBJ,LPMIXERCONTROLDETAILS,DWORD);
1521 UINT WINAPI mixerGetControlDetailsW(HMIXEROBJ,LPMIXERCONTROLDETAILS,DWORD);
1522 #define mixerGetControlDetails WINELIB_NAME_AW(mixerGetControlDetails)
1523 UINT WINAPI mixerSetControlDetails(HMIXEROBJ,LPMIXERCONTROLDETAILS,DWORD);