Can I Stack these two Arduino Shields?
Yes, yes you can. And the beauty is that I can tell without even looking at the Si4735 shield you are using.

How is this possible you ask? The only pins that the AudioShield use are the SPI lines (MOSI,MISO,CLK), a SPI chip select line, and an analog pin for reading the potentiometer. Obviously, the A3 analog pin for the potentiometer can be moved around and put on any other analog pin. The chip select is just an active low digital signal used to determine which SPI device should currently be using the data on the SPI lines (the SPI protocol was designed for a shared bus).

So all in all what that means is for the AudioShield, all of the pins it uses were either designed to be shared, or are not really pin specific and through some super-simple hacks can be moved around. The link you provided actually explains how to move the chip select signal that I talked about to a different pin and the potentiometer hack is basically the same but you have to move it to an analog pin. Of course any code samples you use will have to be adjusted for the new pins you use.

For more information on the arduino pins see here.

For more information on sharing the SPI bus between two devices see here.