Arduino Uno synthesizer - Line Level (audio) output to mixer/amplifier
In theory, the peak signal accepted from your amplifier should be 0.894V p2p if you want to rescpect the common -10dB signal strength/level. I don't know why you would want to have a 5V to 2.5V "voltage divider" in the first place, but in any case if you use that you already limit the current probably enough, it all depends on the resistor values used.

Using the LM386 amplifier is also simply not required at all, since the Arduino is already providing more than enough power. If you want to tune the volume at that stage, simply combine all functions (volume, low pass and current limit) by connecting the 5V PWM output thru a resistor (eg. 4k7) to a POT (eg. 1k) in parallel with the capacitor (eg.2n2) would give you approximately what you want. Then to remove de DC component, add a series capacitor (eg. 1uF) and you're all set. You may still hear a "click" when you connect it to the amplifier though, so keep the faders low when connecting.

Of course you can adapt the values quite liberally, but these values are both safe and should be close to what is needed (cutoff 15kHz, Vpp 0.88V max).