Is there a circular gold sticker on the bottom of the loco? - Should have a six-digit number on it..
What is that number please? - This will give us the approximate manufacturing date.
I think the sound card may need serial commands, due to its' age..
I have just had this with a loco I added a decoder to. If the CS is set for parallel for this loco, then you can only control things by pressing the F1 button repeatedly. - And that is not very reliable!
If the CS is set for serial, then sounds work as they should.
Unfortunately, I am unable to tell you how to set the MTS system to send serial commands.