Last year I had to replace my trusty, nearly 20 year old Mackie 1202VLZ Pro, after it had intermittent static issues, with a new Mackie 1202VLZ4. In my audio booth I had an equally trusty Shure SM7.
About a month ago the audio level coming into the mixer from the mic started to go lower and lower to the point where I was having to raise the audio level on the Mackie more than twice as high as before, which started to introduce hum into the track.
I figured the mic was nearing the end of its life, so I sent it back to Shure for a brand new SM7B. I also ran new mic cable to the booth and switched inputs on the mixer. And today, after trying out all the new stuff, the low level persists.
Any thoughts on why after years and years of similar levels from two reliable audio components I'd be having these issues? All my buttons, switches and gains are the same. And I'm stumped.
Just go through the 'standard' for of testing...
Different Mic (you have done) try a different brand / model etc,
Different inputs, etc etc
Does the signal go through any other gear like compressors / insert points on the desk?
See what happens after those tests, Its unusual for a mic to gradually fade in level.