You really can't beat LTSpice, it's not the fastest spice simulator out there, but it's complete and free, it also has the added advantage that it can both input and output multi channel wave file data. It's definitely not real time but you can feed an audio signal into a circuit input and output any node (even an equation) as a .wav file and analyze it to your hearts content. Never experimented with the maximum sample rate it will let you use. But it at least does 96khz audio. The learning curve is a little higher than a prettified simulator but it's usefulness is significantly higher too.