Hi Stan and anyone else interested. To answer your question, I did not solve the pop problem.
However, I just wanted to show you what all your help and a lot of C programming reading accomplished. I made an HO scale test bed (my railroad is o-scale) and connected 2 'cross-track' sensor pairs and one 'side-by-side' sensor pair to an Arduino. I made a simulated crossing gate operated with a servo motor and connected that along with the MP3 player/amp module/speaker combo also to the Arduino. I wrote (and re-, re-, wrote) a C program to control all. It finally all works. I have enclosed 2 pics of the setup (what a mess of wiring--will try to simplify), and a video of the operation.
Sorry, it turns out that I cannot upload the video. It is 180MB and I get an error when trying to upload as an attachment.
