If your hearing test is within limits at the time of your physical exam you will not need a new test. If your hearing is out of limits that would be another disqualification, but again, once the tympanoplasty is complete your hearing should get better so you would need a new hearing test (from your family physician or the ENT physician thats doing the surgery) and rebutt the disqualification.
It is possible that the tympanoplasty will not take. If that happens, it depends on what SA you are applying to. Any of the sea service academies (USNA, USCGA, USMMA) most likely would not grant the waiver. USMA and USAFA you would have a chance for a waiver.
I'm not trying to discourage you, but I do want you to know what the possibilities are. I've said this to countless applicants and families over the years, if this is what you want, do not take no for an answer. If you get disqualified by DoDMERB and waiver denied by one of the waiver authorities, you can request that the waiver authority take a re-look at your file as many times as it takes. Provide them with new medical information if you have some or provide them and the SA's a personal letter explaining why this is important to you. I have seen cases where I knew there was no way that anyone would ever approve a waiver, yet the applicant was persistant (persistant does not mean rude or abusive) and in the end the waiver was granted.