You could just use an audio ducker or compressor/expander with a mic preamp on the sidechain input.

You plug a microphone into the sidechain which works as an ambient noise sensing mic. On a compressor or ducker that allows an inverse compression ratio you can set it so the volume raises when a loud input signal is heard through the sidechain.

A bit of research is needed around the makers of cheap professional audio processing equipment - behringer for example might have the right item for you.

Obviously a decent DSP based audio processing system will let you programme very specific conditions for how and when volume is altered but this costs a lot.

Roland