The ridiculous solder joint that John points out can cause two problems first the solder can be bridging the gap and cause other problems but who ever did that could have fried the component with that much solder that close to the case. The heat can be conducted into the junction of the device. A good way to kill any solid state device. I am assuming that you know for a fact that the two boards that you mated were good functional boards before you connected them but had a loud buzz in the speaker afterward. One question that has not been asked. "What are you using for a soldering iron" ? Very many soldering irons can have a tip potential when plugged in and measured to a known good ground. This tip potential can be as high as 40v. So hook your VOM to a known good ground and then with the iron plugged in and the meter set on VAC start measuring with your VOM set at something like 200v and if you see nothing turn it to the next lower range when you are finally on the lowest range and see something under 2VAC that iron is likely suitable for modern electronics though 0V is the goal. Never use solder guns on these boards quick heating guns are induction devices and can have a tip potential as high as 40v. John swears by his Hakko Iron I have a professional Weller esd safe iron that lives on my bench with a 0v tip potential and a couple of cheap chinese irons that read 0V as well in my tool boxes. A bad iron can fry components faster than you can change them. Especially ICs and FET transistors. j
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