You should do the x-ray as that will tell you if she has a trachea issue. If its kennel cough the medications your vet gave you should help with that.
You can read about kennel cough here (its a virus similar to a human cold)
www.marvistavet.com/html/kennel_cough.html
If it is a trachea issue you may need to help your pet with losing some weight if she is plump.
The fact that the heart and lungs sound good is great as often the heart has a hard time if the airway is blocked and that can also lead to pneumonia in the lungs as fluid builds up.
You can read about trachea problems here
www.marvistavet.com/html/body_tracheal_collapse.html
I'd wait to see what the x-ray tells you or if the meds work to resolve this before getting a second opinion.
Hope this helps you!
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