Thank you for posting on the ANA Forums!
How long ago was the concert? It generally takes a long time for an acoustic neuroma to develop.
Have you had your second MRI? Usually the second MRI can reveal how slow or how fast the acoustic neuroma is growing.
If you have had your second MRI, and if it reveals a very slow growing acoustic neuroma, and if the concert was eight months ago, and it would take 3 years for the tumor to grow to its current size, then that would indicate that the acoustic neuroma had started long before the concert.
Your ENT might be able to give you an idea, based on the growth rate of the tumor and its current size, approximately when your acoustic neuroma began. And, if it was well before your concert, then you would know that the two events are not related.
Unfortunately, without knowing the size of your tumor, the growth rate of your tumor, and the date of your concert, none of us on this forum would know if there is a connection with the concert or not.
While loud noises can be connected to tinnitus, I have never heard of someone developing an acoustic neuroma to a concert before, but that does not mean that it could not happen. Yet, I would be slow to blame the concert until I had further information.
Best wishes on your journey!