Hi,
Ground planes are very welcome on RF circuit boards, but tend to be a real PITA for audio design due to spreading ground noise and making massive ground loop hum.
Ground planes are "shorted loops" which confine radio signals on RF projects. Those same loops cause hum at audio.
If you look at double sided commercial boards, if they use one side as a ground plane, you'll see the traces etched so they act as individual wires all going to a single ground point, usually at the lowest impedance point (at the PSU). Golden Tube IMO, was the finest example of this in a commercial build.
Then you get some not-so-good groundplanes. Some of L'Audio's early digital/tube hybrid models is an example of how to do it very badly.
For my own designs, I've dispensed with using any groundplanes and never have had hum/noise issues. So if when I get double-sided board less expensive than single sided due to a sale, I just etch the one side completely off, or use it as an etched "silkscreen" for terminal point lables

Cheers!