Why is the LED attached to pin 10 on and how can I get it to turn off
That's odd. Are you sure it's the LED on pin 10, and not the one on the other end, on pin 1?  

Take a look at page 7 of the datasheet. There's a resistor ladder network creating a series of voltages to compare to, starting with the reference voltage for the LED on pin 10. If the input voltage is higher than this \$V_{REF}\$ the output of the comparator will go low, and the LED will be on. For the other levels the comparator uses a lower voltage each time. So if only the LED on pin 10 is on it would mean that the input voltage is higher than the reference, but NOT higher than the other comparison voltages, which are lower. This just doesn't make sense.
It would make sense if it were the LED on pin 1, then it could be that you have a DC offset in your signal, which is higher than the first threshold.  

edit
I did notice that you use the LM3915 in dot mode (which I commented as odd for a VU-meter; you would rather use bar graph mode there), but I didn't connect this to my previous observation. Of course you'll see only one LED in dot mode! If the LED on pin 10 is the only LED you can get lit your input signal may be too high. Try to short the input to ground, and, alternatively, connect pin 9 to pin 3 and see what happens. If my hunch about a too high level is correct you should see all LED on.