Following Roman's idea:

But this is for Unipolar motors (5 wires):

With 4 audio channels to 2 sets of small USB speakers (2 stereo amps / 4 speakers ) wouldn't it achieve “super smooth micro stepping”?

https://en.wikipedia.org/wiki/Stepper_motor#Phase_current_waveforms

https://en.wikipedia.org/wiki/Stepper_motor#Microstepping

Mensaje telepatico asistido por maquinas.

From: Martin Peach <chakekatzil@gmail.com>
Sent: Sunday, August 14, 2016 5:52 PM
To: Andy Farnell
Cc: Lucas Cordiviola; pd-list@lists.iem.at; ray Y
Subject: Re: [PD] stepper control

On Sun, Aug 14, 2016 at 1:34 PM, Andy Farnell <padawan12@obiwannabe.co.uk> wrote:

On Sun, Aug 14, 2016 at 08:16:52AM +0000, Lucas Cordiviola wrote:

> Also please be careful and be aware that there is risk of
> damaging things.
>
> Consider this a risk to the amp, do it with a low power one,
> probably with one of those inside little usb speakers.

Any inductive load will give a back EMF on the negative going edge
of the pulse. Look at stepper motor driver circuits and you will
see that protection diodes are used. Push-pull audio amps aren't
really designed for this kind of punishment, they are linear,
whereas stepper drivers are designed to switch a clean pulse.
That said, for small motors you will get limited success, so its
a useful hack to know.

And most cheap audio amps won't go bipolar, the 'negative' output is always 0 Volts. You would need 4 channels of sound to do it.

But if the OP already has an Arduino and an L298 it just makes sense to program the Arduino to step the motor and use the serial port to tell it when to step or at what speed to run. If you want better sync, try sending a square wave audio signal from the Pd patch to the Arduino and program the Arduino to use that as the stepper clock. (You would need two channels of audio, one for each direction.)

Martin