The frequency of a 'beat' is completely different to what we normally refer to as the frequency of a sound. The beat's frequency relates to how quickly the amp

The frequency of a 'beat' is completely different to what we normally refer to as the frequency of a sound. The beat's frequency relates to how quickly the amplitude of the sound wave varies. The frequency of a tone relates to how quickly the pressure waves oscillate back and forth. Besides, if a combination of sounds outside the audible spectrum DID combine to produce audible sounds (maths says they don't, but maybe non-ideal properties of air etc mean they might?), the resultant audible sounds would be picked up by the recording equipment anyway! So you'd never need to record the inaudible source sounds, just the resultant audible bit.