I'm trying to set GNU Radio as an audio processor for a little community radio in my town.
I've already installed GNU Radio and it's working, but I'm not a sound engineer, so I need some help.
This is my installation:
MIC & Music Player  ----> Mixer ----> GNU Radio ---> FM Emitter
I need to know what filters and modules to set to improve sound in this workflow.
Could any of you give me an outline of what GNU Radio modules to use?
Since the aim is to improve sound quality in our little community radio, the right way to achieve it is to use an audio processor software, as @KevinReid said.
For the records, one possible solution is use this schema with Jack:
MIC & Music Player ----> Mixer ----> PC with audio processor ---> FM Emiter
The PC with audio processor is a GNU/Linux based PC with Jack as sound server and Calf Jack Hub (calf.sourceforge.net) as audio processor.
Steps:

Install jack, qjackctl and calf.
Open qjackctl and start jacks server
Open calf and set filters you want (eq, limiter, compressor, etc.)
Set connections so you take the input, send it through filters, put it into the output (i.e. headset connector or lineout)

That's all. All this can be done by command line, at startup, etc... but this shows the main idea.
GNU Radio on Community radios