and finally as a possible update to John's method...
Sub DisableAutoAdvanceForAllSlides()
    Dim opres As Presentation
    Dim strFile As String
    Dim strFolder As String
    Dim strSpec As String
    Dim osld As Slide
    ' Specify the file pattern to find PowerPoint files
    strSpec = "*.pp*"
    ' Specify the folder containing the PowerPoint files
    strFolder = Environ("USERPROFILE") & "\Desktop\PPTFILES\"
    ' Get the first file matching the pattern in the folder
    strFile = Dir$(strFolder & strSpec)
    ' Loop through all matching files in the folder
    While strFile <> ""
        On Error Resume Next   ' Enable error handling for opening files
        Set opres = Presentations.Open(strFolder & strFile)
        On Error GoTo 0   ' Disable error handling after attempting to open
        ' Check if the presentation was opened successfully
        If Not opres Is Nothing Then
            ' Loop through each slide in the presentation
            For Each osld In opres.Slides
                With osld.SlideShowTransition
                    ' Disable automatic advance
                    .AdvanceOnTime = False
                    .AdvanceAfterTime = 0
                    ' Ensure advance on mouse click is enabled (optional, but good practice)
                    .AdvanceOnClick = True
                End With
            Next osld
            ' Save the changes opres.Save
            ' Close the presentation
            opres.Close
        Else
            ' Inform the user if a file could not be opened
            Debug.Print "Could not open file: " & strFolder & strFile
        End If
        ' Get the next file matching the pattern
        strFile = Dir$()
    Wend
    MsgBox "Automatic slide advance has been disabled for all PowerPoint files in the specified     folder.", vbInformation
End Sub