One .WAV file is used for all reminders. For anything else you'd have to
code your own reminder interceptor event handler and then call the Win32 API
procedure that plays a sound, using whatever .WAV file you wanted based on
the type of the item that fired the reminder.

See http://www.outlookcode.com/d/code/sendreminder.htm for a code example of
intercepting the reminder event. The Win32 API call you need to use to play
a sound looks like this:

Public Declare Function sndPlaySound Lib "WINMM.DLL" Alias _
 "sndPlaySoundA" (ByVal lpszSoundName As String, ByVal uFlags As _
 Long) As Long

Public Const SND_SYNC = &H0
Public Const SND_ASYNC = &H1
Public Const SND_NODEFAULT = &H2
Public Const SND_LOOP = &H8
Public Const SND_NOSTOP = &H10

Public Sub PlayReminderSound()
 Dim SoundName As String
 Dim wFlags As Integer
 Dim X As Integer

 On Error Resume Next
 SoundName = g_strReminderSound 'file name and path to .WAV file
 If SoundName <> "" Then
   wFlags = SND_ASYNC Or SND_NODEFAULT
   X = sndPlaySound(SoundName, wFlags)
 End If

End Sub