L.O.Guvna
Hello & Welcome to the Forum!

The whistle not working is a common problem on these MPC era locomotives, there might be a work around, but I'm not aware of one.
The sound board in the tender is prone to shorting out when the insulation between it and the tender frame wears out.
I suggest you open your tender and replace the insulation if you haven't already done so. I used a piece of 1/8th inch extruded foam and a piece of card stock to replace the original insulation.

Parts:
Dr Tinkers Trains
http://www.ttender.com/
Brasseur Electric Trains
http://brasseurelectrictrains.com/
The Train Tender
http://www.ttender.com/

There are other suppliers, but these are the three I've had the best results with.

Happy Rails!
Dave