From a looks point of view my 1520 looks brand new, but it is starting to feel it's age, on calls the microphone cuts out (Well lets say "I go very quite for a bit then come back") for a few seconds on maybe every 10th call or so, my battery isn't lasting as long as it used to, again it's variable most days fine, but others it's lasting 50% less with no apparent reason, and web surfing hasn't ever been the best experience I have ever had.
I am starting to think there will be life after 1520 and it is probably coming sooner than I had anticipated, my daughter is getting married in May, I really don't want to spend any money before then so if the 1520 was to die now I think I would get a 650 to get by with, but then the hunt for the 1520's real replacement begins with the 950XL, the Surface phone or the HP Elite being options at the moment.
But back to your original question, I have seen a few comments regarding the microphone on 1520's having the odd issue and there is at least one video of how to replace it, but I understand what you say about opening them up and if you replace the mic, do you replace the battery etc.
Does the age of the phone warrant it now, I think I will let my 1520 die un-molested and with dignity
It can go and continue it's life next to it's much loved predecessor the small but mighty HTC HD2 (Lasted 4 years) in the drawer I call "Almost dead, but not really, phone heaven".