How do I protect a unipolar ADC input against a bipolar signal?
You're so close:
just use a Schottky diode to gnd that's in reverse bias during normal operation, and in forward bias if the input voltage is negative. Depending on your diode, that would clip to -0.1 to -0.2 V, ca.
The only &quot;normal&quot; usage current would be the reverse leakage current, and that shouldn't be very high if you use an appropriately sized diode.
It is also worth noting that any diode in reverse bias is in fact a bit of a capacitor (that's used, in fact, in voltage-controlled RF filtering/attenuation circuitry), but the capacity here should be very small.
It might make sure to consider whether you can add a bit of series resistance to your audio inputs without reducing the quality, just to limit the current through that diode.

Are there any ideal diode circuits that can be used for this purpose?

Yes, but you'll in the end need a negative supply voltage to make these work. There's some op-amps that are &quot;beyond rail-to-rail input&quot;, by using an internal charge pump to generate a small negative supply voltage. But it really doesn't sound like you'll need this – a diode should be fine.