Well, speaking from my own experiences, when my signs of PTSD first showed up, I had an auditory flashback. I only had one single sentence to go on, but it made me feel like I was going crazy. It was only after I saw my psychiatrist and he dragged out my trauma (my main one anyway); that I was diagnosed with PTSD and started getting more flashbacks.
Flashbacks come in many forms. They can come in auditory forms (a sound connected to the trauma- a sentence from the traumatic event spoken by someone, sounds of metal or such if it was a car crash. etc. etc)
They can come in visual forms- where you SEE the event happening again. (This is the most well-known usage of the term flashback, usually when people say flashback, it is assumed they are talking about this form, but not everybody gets that form.)
They can also come in body memories. These are often flashbacks which come to people traumatized by sexual abuse or physical abuse. For example, I was a victim of child abuse. When I get body memories, the place where I was hit actually hurts, and it turns red. These are terrifying flashbacks, and if you don't realize it before, they may not even be considered by a person to BE flashbacks.
If you don't get these kinds of flashbacks, you don't have PTSD. It is REQUIRED under the diagnostic criteria that a) The event must have happened at least 3 months ago. b) You must have experienced intense fear and horror at the time of the event. and c) You must get recurrent flashbacks in which you feel the event is happening all over again.
Hope this helps.
Jen.