Try pressing F6 a couple of times, that sets the volume. If I recall correctly, (don't quote me) the bell and horn volume are independent of the F6 volume control.
Were you using Programming On Main or a programming track? I have only limited experience with MTH DCC, but I found it to be a little finicky. It took me several attempts to shut off the bell and horn in my B-unit using Programming On Main.
Setting CV55 to 55 will do a full factory reset.