There are several ways to do it;
you could use a dedicated mic amplifier (which may implement features like AGC). An example is **broken link removed** but it may be hard to find.
Or, you could use a better op-amp, e.g. see the circuit **broken link removed** which uses
a low cost fairly easy to find op-amp, or use any of **broken link removed**.
None of them are particularly exotic to find, and will give good performance for an intercom.
Or, you could use a transistor preamp circuit, e.g. the one in the thread mentioned earlier.