See the first couple of pictures herehttp://www.gammon.com.au/forum/?id=10894
Look at the signal in oscilloscope mode (does Audacity let you look at the file like that?) zoomed in enough that you can determine where the 0s & 1s are that make up the bytes. If a data format of 8N1 is being used, you need to discern 10 bits/byte: 0 for start bit, 8 high/low data bits, 1 for a stop bit.
A sampling rate of 48000Hz may or may not be fast enough. If the data rate is 4800, then you have 10 samples per bit, might be discernible. If the datarate is 9600, then you only have 5 samples per bit and it could be harder.
On the other hand, you could just try different data rates with the serial port, and see if the hardware can sync up by itself:
in void setup():
in void loop():
Serial.print(Serial.read(),HEX); // or Serial.println, to get one character per line
Try 2400, 4800, 9600, 19200, see if anything recognizable comes out.
If not, you're stuck with trying to analyze the data.