Speaking as a qualified veterinary surgeon, I am surprised to hear that a diagnosis of hyperadrenocorticism (HAC or Cushings Syndrome) was made without any blood tests being performed.
To diagnose this disease I would expect a minimum database of haematology and blood biochemistry, and then either an ACTH stimulation test and/or a low-dose dexamethasone suppression test to have been carried out. Only if the results of these, and the clinical signs (including some or all of the following: excessive drinking, excessive urination, enlarged liver, changes to appetite, changes to skin and coat, associated diseases) were suggestive should medication for HAC be started.
As for the heart murmur, these can be very dynamic things which can change over time - depending on their seriousness and underlying cause they can be more evident one day thatn another and in some cases can disappear. It may have been there, and the medication may have helped... but, there is a chance that that was also a misdiagnosis.
If you are worried that the previous vet was incompetent, or worse, unscrupulous, you should contact The RCVS (Royal COllege of Veterinary Surgeons) who regulate our profession.
There website is: www.rcvs.org.uk
I would expect that they would contact the previous vet and ask to see the notes he made on his original diagnosis...
Hope this helps.
It would be interesting to hear the outcome....