Most of today's ESCs, brushless and brushed type, already have the capability to vary the chopping frequency. When you power up the ESC you can usually hear tones generated in the motor which are used to indicate the status. Often multiple tones are used to indicate different modes. It would actually be possible to play music through the motor this way (not very good though
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I can briefly describe the way in which sine drive can be accomplished in a brushless ESC. Basically it is equivalent to a 1-bit digital-to-analog conversion with the motor itself acting as the low pass filter at the output stage.
In a standard brushless ESC the phase currents are chopped at a fixed frequency for part throttle operation with the on/off duty cycle varying as a function of the throttle position. At full throttle the current is not chopped at all and the phase switches remain on constantly for the duration of each particular phase.
In a sine drive the current is chopped in a manner where the chopping frequency is modulated so as to approximate a sine wave in the time domain. In the sine drive motor controller there would be a digital representation of a sine wave encoded in its firmware to use as the 'pattern'. This is similar to the way in which a digital audio system can create analog music from a binary (ones & zeros) data source.
I should warn you though that although this is the most efficient drive from the motor's perspective; that is is results in a cooler running motor, there are increased switching losses in the controller at full throttle. So the overall net system efficiency might not necessarily be better.
-gerald