E.B.
Of course we can't give you medical advice on here, and we don't know your son's history (the cause of the murmur, what his internist is saying about his gallbladder, etc), but we possibly can give you a couple of things to think about. Sometimes when you're worrying about your child, you can overlook important questions to ask.
First, make sure that every doctor is fully informed about your son's medical history. Don't assume that a doctor automatically has reviewed his heart murmur records or any cardiology records. Ask "have you taken his heart murmur into consideration?" or "have you spoken with his cardiologist?".
Don't be hesitant to ask the doctor who says "I think we should remove his gallbladder" a question about how that will impact his heart. And have a notepad ready to write down suggestions/answers/notes, etc. If a doctor says a word you aren't familiar with, ask the doctor or a nurse to spell it. I often write down crazy long words as they sound. Then I type them into google, and google corrects the spelling. For example, if you can't spell pneumonia, just write noomonia and google will ask you "did you mean pneumonia"?". It's pretty handy.
Don't be afraid to seek a second opinion. Be respectful and polite, if you really feel that a second opinion might be necessary.
Ask your doctor "if this was your child, what decision would you make" if surgery is only suggested, not necessary.
Keep careful records. Have a file ready with your son's most pertinent records available. This should include any xrays, gallbladder tests, heart tests. Have your son's doctors' contact information readily available. It's amazing how easy it is, in a time of stress, to forget Dr. What's-his-name, or when you saw his primary care physician last. Have these in a handy reference place so you can be ready with answers like doctors' names, addresses/phone numbers, last appointment, etc.
If your son takes medication of any kind, ask the pharmacy for a list of dosages and names. Also, if your son takes any vitamins/supplements/energy drinks regularly, write those down or take pictures of the bottles. Be thorough!!!