Using PWM pins on the UNO to generate Stereo Sound
You can generate &quot;stereo sound&quot;, but I don't think you'll like it.  The PWM frequency is only 490 or 980 Hz, which is going to be plainly audible unless you filter the output very severely.  So you'll only be able to produce low frequency sounds, about like what comes out of the woofer speakers in your home stereo.  In other words, nothing like music.  Might be OK for certain experiments, but nothing you'd want to listen to for pleasure.
As far as additional circuitry, you'll need to convert the DC outputs of your two low-pass filters to AC before you try to drive headphones.  This requires a big decoupling capacitor for each channel. If you are going to drive a separate amp, you might be able to skip the capacitors since most amps have them built into the inputs.