Frankly, I wouldn't even bother with the FileSystemObject - I'd simply use a folder picker, then use the Dir function to retrieve only .ppt* files. Avoids a lot of the circumlocution the current code is doing. For example:
Sub ConvertPresentations()
Dim strSrcFldr As String, strTgtFldr As String, strFlNm As String, ppPrs As Presentation
    strTgtFldr = GetFolder("Choose an OUPUT folder")
    If strTgtFldr = "" Then Exit Sub
    strSrcFldr = GetFolder("Choose an INPUT folder")
    If strSrcFldr = "" Then Exit Sub
    strFlNm = Dir(strSrcFolder & "\*.ppt", vbNormal)
    While strFlNm <> ""
        Set ppPrs = Presentations.Open(FileName:=strSrcFldr & "\" & strFlNm, ReadOnly:=True)
        With ppPrs
          .SaveAs FileName = strTgtFldr & "\" & Split(strFlNm, ".ppt")(0) & ".mp4", FileFormat:=ppSaveAsMP4
          .Close
        End With
        strFlNm = Dir()
    Wend
    Set ppPrs = Nothing
Application.ScreenUpdating = True
End Sub
 
Function GetFolder(strMsg As String) As String
    Dim oFolder As Object
    GetFolder = ""
    Set oFolder = CreateObject("Shell.Application").BrowseForFolder(0, strMsg, 0)
    If (Not oFolder Is Nothing) Then GetFolder = oFolder.Items.Item.Path
    Set oFolder = Nothing
End Function