diff --git a/lib-src/portmidi/pm_win/pmwinmm.c b/lib-src/portmidi/pm_win/pmwinmm.c
index 2de8109a0..22961d5a0 100644
--- a/lib-src/portmidi/pm_win/pmwinmm.c
+++ b/lib-src/portmidi/pm_win/pmwinmm.c
@@ -37,16 +37,16 @@
 #define MIDI_EOX        0xf7
 
 /* callback routines */
-static void CALLBACK winmm_in_callback(HMIDIIN hMidiIn,
-                                       WORD wMsg, DWORD dwInstance, 
-                                       DWORD dwParam1, DWORD dwParam2);
-static void CALLBACK winmm_streamout_callback(HMIDIOUT hmo, UINT wMsg,
-                                              DWORD dwInstance, DWORD dwParam1, 
-                                              DWORD dwParam2);
+static void CALLBACK winmm_in_callback(
+   HMIDIIN hMidiIn, UINT wMsg, DWORD_PTR dwInstance, DWORD_PTR dwParam1,
+   DWORD_PTR dwParam2);
+static void CALLBACK winmm_streamout_callback(
+   HMIDIOUT hmo, UINT wMsg, DWORD_PTR dwInstance, DWORD_PTR dwParam1,
+   DWORD_PTR dwParam2);
 #ifdef USE_SYSEX_BUFFERS
-static void CALLBACK winmm_out_callback(HMIDIOUT hmo, UINT wMsg,
-                                        DWORD dwInstance, DWORD dwParam1, 
-                                        DWORD dwParam2);
+static void CALLBACK winmm_out_callback(
+   HMIDIOUT hmo, UINT wMsg, DWORD_PTR dwInstance, DWORD_PTR dwParam1,
+   DWORD_PTR dwParam2);
 #endif
 
 extern pm_fns_node pm_winmm_in_dictionary;
@@ -666,10 +666,10 @@ static PmError winmm_in_close(PmInternal *midi)
 /* Callback function executed via midiInput SW interrupt (via midiInOpen). */
 static void FAR PASCAL winmm_in_callback(
     HMIDIIN hMidiIn,    /* midiInput device Handle */
-    WORD wMsg,          /* midi msg */
-    DWORD dwInstance,   /* application data */
-    DWORD dwParam1,     /* MIDI data */
-    DWORD dwParam2)    /* device timestamp (wrt most recent midiInStart) */
+    UINT wMsg,          /* midi msg */
+    DWORD_PTR dwInstance, /* application data */
+    DWORD_PTR dwParam1,   /* MIDI data */
+    DWORD_PTR dwParam2)   /* device timestamp (wrt most recent midiInStart) */
 {
     static int entry = 0;
     PmInternal *midi = (PmInternal *) dwInstance;
@@ -688,7 +688,6 @@ static void FAR PASCAL winmm_in_callback(
          * hardware interrupt? -- but I've seen reentrant behavior 
          * using a debugger, so it happens.
          */
-        long new_driver_time;
         EnterCriticalSection(&m->lock);
 
         /* dwParam1 is MIDI data received, packed into DWORD w/ 1st byte of
@@ -697,8 +696,6 @@ static void FAR PASCAL winmm_in_callback(
             in [ms] from when midiInStart called.
            each message is expanded to include the status byte */
 
-        new_driver_time = dwParam2;
-
         if ((dwParam1 & 0x80) == 0) {
             /* not a status byte -- ignore it. This happened running the
                sysex.c test under Win2K with MidiMan USB 1x1 interface,
@@ -709,8 +706,8 @@ static void FAR PASCAL winmm_in_callback(
             PmEvent event;
             if (midi->time_proc)
                 dwParam2 = (*midi->time_proc)(midi->time_info);
-            event.timestamp = dwParam2;
-            event.message = dwParam1;
+            event.timestamp = (PmTimestamp)dwParam2;
+            event.message = (PmMessage)dwParam1;
             pm_read_short(midi, &event);
         }
         LeaveCriticalSection(&m->lock);
@@ -731,7 +728,7 @@ static void FAR PASCAL winmm_in_callback(
         /* assume yes and iterate through them */
         while (remaining > 0) {
             unsigned int amt = pm_read_bytes(midi, data + processed, 
-                                             remaining, dwParam2);
+                                             remaining, (PmTimestamp)dwParam2);
             remaining -= amt;
             processed += amt;
         }
@@ -1327,8 +1324,9 @@ static void CALLBACK winmm_out_callback(HMIDIOUT hmo, UINT wMsg,
 #endif
 
 /* winmm_streamout_callback -- unprepare (free) buffer header */
-static void CALLBACK winmm_streamout_callback(HMIDIOUT hmo, UINT wMsg,
-        DWORD dwInstance, DWORD dwParam1, DWORD dwParam2)
+static void CALLBACK winmm_streamout_callback(
+   HMIDIOUT hmo, UINT wMsg, DWORD_PTR dwInstance, DWORD_PTR dwParam1,
+   DWORD_PTR dwParam2)
 {
     PmInternal *midi = (PmInternal *) dwInstance;
     midiwinmm_type m = (midiwinmm_type) midi->descriptor;