I was finally able to come up with a viable solution that required explicit typing of all of the variables (including using a separate variable for the .toString() operations) as seen below :
public function download(event:MouseEvent):void
{
//Build a simple file to store the current file
var outputFile:FileReference = new FileReference();
//Perform a function to build a .wav file from the existing file
//this returns a ByteArray (buffer)
downloadBuffer = PrepareAudioFile();
//When accessing the actual length, this needed to be performed separately (and strongly typed)
var bufferLength:uint = downloadBuffer.length;
//The string process also needed to be stored in a separate variable
var stringLength:String = bufferLength.toString();
//Use the variables to properly concatenate a file name
var fileName:String = dstringLength + ".wav";
//Save the file
audioFile.save(downloadBuffer, fileName);
}
It's bizarre that these had to explicitly be stored within separate values and couldn't simply be used in-line as demonstrated in the other examples.