Highest frequency that can be generated with SPWM?
This document from Texas Instruments goes into some detail on designing a digital to analog converter using pulse width modulation.

It references a particular TI microprocessor to generate the PWM signal, but the principles should apply to any PWM DAC.

A (drastically simplified) summary of that TI document:


The maximum frequency of the sine wave depends on the frequency of the PWM signal.
The maximum output frequency depends on the required purity of the generated sine wave (the crappier the output is allowed to be, the closer you can get to the PWM frequency.)
How close to the PWM frequency you can get depends on the needed quality of the sine wave and how good  your low pass filter is.   A good sharper filter will remove more of the PWM frequency, allowing the output signal to approach the PWM frequency.


So, it is a trade off that you have to make to meet your other requirements.

The document goes into a lot of detail on how to make the compromises.



A couple of examples combining the TI notes and the Arduino:

Aiming for 5 bits equivalent using the highest PWM rate I found for the Arduino (62500Hz,) I estimate a maximum output sine wave of 195Hz.

Aiming for 14 bits equivalent using 62500Hz PWM, I figure about 4Hz as the maximum output sine wave.  Note that this is only approaching the precision of a typical sound card (16 bits) and only if your output frequency stays below 4Hz.

Note:  I have ignored the effects of the output filter in the above examples.