Is it okay to use rechargable batteries in a microphone? I didn't see anything in the manual (AT897) that said I couldn't. But I just wanted to double check with people here.

There is nothing in the manual about it, but I'd sure never consider it. Times have changed, but rechargeables still have all sorts of issues. For the 1.00, I'd continue using a standard alkaline battery.
Some mic companies, and still some wireless companies, used to void the warranty if you use rechargeables.
Keep in mind that as the battery wears down, so does your quality of output.

I agree I wouldn't recommend it as rechargables have a slightly lower load voltage but a much higher current ability.Some electronics use the lower current capacity of alkalines in their design and you can fry certain devices by using rechargables!!!!