There might be spaces in the FileName and Char(34) might not work.
Put the " in the command.
FileName = Button1.Text
mciSendString("open """ & FileName & """ alias myDevice", Nothing, 0, 0)
mciSendString("play myDevice", Nothing, 0, 0)
FileName = Button2.Text
mciSendString("open """ & FileName & """ alias myDevice", Nothing, 0, 0)
mciSendString("play myDevice", Nothing, 0, 0)