Probably wouldn't really be considered old-school soft circuits but probably more of a hybrid, electronics in a softshell case. I still have a bobbin of conductive thread I need to do something with. Note that this was a first attempt in trying to do a mod with stuff I had on hand but I will offer suggestions on what you should do to get a more satisfactory result.
Ruby's partner in crime Mho the Resistor plushie has been modded as an analog resistor value decoder.
Step 1: Stocking stuffers...
A sound reactive LED circuit. I do not know how to put one together from scratch, well I could try but I'm lazy. I am also cheap. Sourcing parts can also take a lot of time. I was in Radio Snacks one day and saw these Velleman Sound to Lights Electronics Kits MK103RS on clearance. I don't think they stock this one anymore. I got two of them because you know you always have to have backup in case you mess up one. Look up "color organ" for these types of circuits.
These beginner kits are great to see if you have the skills to solder and to see if you can follow simple directions.
I am going to mod those kits a little bit to make it work for my application. For ease of placement in the plushie, I wanted to extend the leads of the battery pack and microphone. I also wanted to swap out all the red LEDs that came with the kit and replace them with brighter white LEDs.
Soldering is fun. Until you burn yourself. Learn how to solder safely.
You also need a bit of sewing skillz. Yup, you need to do a little bit of plushie surgery to implant the bionic devices.
If you are a sewist (seems people really didn't like being called sewers) you should have a seam ripping tool, needle and thread.
An x-acto knife or utility knife would suffice too.
Pointy things are pointy and sharp things are sharp. Pointy sharp things are extra pointy and sharp. Be careful.