Playing 2 freq!

Not really Epilot. A common speaker is a single transducer and can transmit energy from about 30hz all the way up to and past 20khz. I'm not sure why you find the multiple frequency concept so hard to understand. In order to understand anything on the frequency domain it has to be integrated over time. The number of samples and the length of time over which they're taken determine the ability to perceive difference in frequencies. It's easiest to understand in that if you have only one sample (5 volts) you have NO reference to base a frequency off of. If you have two samples over time, say 5 volts and then one second later 1 volt you then have a reference to base a possible frequency off of. You need to read up on basic quantization. Even an 'analog' circuit has the same limitations.

or even better, go to a library and find a book on signals & systems. What you seem to be struggling with is the concept that one signal can be the superposition (sum) of multiple other signals, particularly sinusoidal signals - that any signal can be represented as the superposition of some number of sinusoidal signals, which gives rise to the concept of the 'frequency content' of a signal. Just because you superimpose two or more sinusoidal signals and get a weird-shaped waveform doesn't mean that anything has changed or the original signal information is lost - all of the information is still there and the two superimposed signals can still be extracted by filtering. This is why you can sum two sinusoidal signals, play the resulting signal through a speaker, and your ear will hear both tones.

You mean three tones =)

Two frequencies through a speaker make two frequencies, not three.
If the amplifier or speaker has distortion then there will be four or more frequencies, not three.

Oddly enough I got the terms mixing and summing mixed up <smirk> Keeping it all straight is a pain.