MTH streetcar with protosound

I have several MTH streetcars with protosound that I restored for holiday gifts and donations to our display at the local museum. Put one on the test track to make sure it was working correctly and a few sparks flew out due to not checking to see the wheels were correctly set. After that every time I put voltage to the track the engine speaker would make a loud humming sound for a minute or two and then would operate ok. Tried the reset procedure with no good result and finally replaced the new BCR red battery with a twelve volt alkaline and the humming sound is gone and protosound is working. Checked the BCR battery voltage and it only reads six volts. Looks like I burned up the battery when I shorted the streetcar out and will order a new one but I don't understand why I didn't burn up the circuit board. Thanks, I am a postwar operator and don'r have much experience with MTH. Thanks, Bill Makel