I just remembered something.
My boards don't have an onboard voltage regulator. I use an external regulator or USB power to supply the 5V I need. Or I power it directly from 3 AA batteries. This means I couldn't add a linear regulator for my audio, because to supply 5V to the audio circuit I'd need to supply the regulator with more than 5V.
I suppose I could create a servo control module of some kind which uses an external power source, but the whole point of this board is to be as small as possible and be a light, sound, and motion controller. A separate servo module would likely not fit in a small handheld prop with servos.
Any other ideas for how I could prevent noise from getting into my audio circuit? Inductors where the digital power and ground planes connect to the analog ones perhaps? I've seen inductors in some papers I've looked at. But my budget and space constraints didn't really allow for them at the time and I thought I could get away without them. Is it possible to get really tiny inductors?
Oh, and the reason I used an external regulator was because late in my design process I discovered the linear regulator I was going to use wasn't going to be able to supply more than 2A without melting my board down and I didn't have space for a heat sink, or the money, space, or time to add a switching regulator, so I ended up purchasing a bunch of fixed 5V switching regulators to get the job done.