I'm fairly new to electronics. I'm designing a simple board with a left and right channel audio input, and two sets of left and right channel outputs. A 16Mhz AVR microcontroller controls two relays (one for each output), which allow each output to be toggled independently. The signals will be line level audio feeding into audio amps, so very little current should be flowing through the signal lines.
In order to minimize noise on the audio signal, what is the best way to handle grounding? Should I have two isolated ground planes, one for signal and one for digital circuitry, or is having one ground plane with the components laid out so that the microcontroller switching current doesn't flow near signal lines sufficient?
Secondly, should I include a relay to switch the signal grounds, or leave them connected? Are there any potential issues with leaving the two output grounds connected, even if only one output is on?